Semiconductor device and method for manufacturing the same

ABSTRACT

A highly reliable semiconductor device is provided. A semiconductor device is manufactured at a high yield, so that high productivity is achieved. In a semiconductor device including a transistor in which a gate electrode layer, a gate insulating film, an oxide semiconductor film containing indium, and an insulating layer provided on and in contact with the oxide semiconductor film so as to overlap with the gate electrode layer are stacked and a source electrode layer and a drain electrode layer are provided in contact with the oxide semiconductor film and the insulating layer, the chlorine concentration and the indium concentration on a surface of the insulating layer are lower than or equal to 1×10 19 /cm 3  and lower than or equal to 2×10 19 /cm 3 , respectively.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device and a method for manufacturing the semiconductor device.

In this specification, a semiconductor device generally means a device which can function by utilizing semiconductor characteristics, and an electro-optical device, a semiconductor circuit, and electronic equipment are all semiconductor devices.

2. Description of the Related Art

Attention has been focused on a technique for forming a transistor using a semiconductor thin film formed over a substrate having an insulating surface (also referred to as a thin film transistor (TFT)). The transistor is applied to a wide range of electronic devices such as an integrated circuit (IC) or an image display device (display device). A silicon-based semiconductor material is widely known as a material for a semiconductor thin film applicable to a transistor. As another material, an oxide semiconductor has been attracting attention.

For example, a transistor including a semiconductor layer which includes an amorphous oxide (IGZO-based amorphous oxide) containing indium (In), gallium (Ga), and zinc (Zn) is disclosed (see Patent Document 1).

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.     2011-181801

SUMMARY OF THE INVENTION

Improvement in reliability is important for commercialization of semiconductor devices including transistors that include an oxide semiconductor.

However, a semiconductor device includes a plurality of thin films each having a complicated structure, and is manufactured using a variety of materials, methods, and steps. Therefore, an employed manufacturing process may cause shape defects or degradation of electric characteristics of a semiconductor device which is to be provided.

In view of the above problem, it is an object to provide a highly reliable semiconductor device which includes a transistor including an oxide semiconductor.

It is another object to manufacture a highly reliable semiconductor device at a high yield to achieve high productivity.

In a semiconductor device including an inverted-staggered transistor with a bottom-gate structure in which an insulating layer functioning as a channel protective film is provided over an oxide semiconductor film containing indium, a surface of the insulating layer and the vicinity thereof are prevented from being contaminated by scattering of residues generated in an etching step for forming a source electrode layer and a drain electrode layer.

A gas containing chlorine is used in the etching step for forming the source electrode layer and the drain electrode layer. However, when the oxide semiconductor film containing indium is exposed to the gas containing chlorine, reaction occurs therebetween to generate residues. Further, when the residues are scattered, the residues exist not only on the oxide semiconductor film but also in another portion. In particular, in the case of the residues on the surface of the insulating layer and in the vicinity thereof between the source electrode layer and the drain electrode layer, they cause degradation of electric characteristics of a transistor, such as an increase in leakage current.

The residue includes, for example, a compound containing indium and chlorine. The residue may further include a metal element other than indium contained in the oxide semiconductor film (e.g., gallium or zinc) or an element other than chlorine used in the gas containing chlorine (e.g., boron).

In one embodiment of the invention disclosed in this specification, after formation of a source electrode layer and a drain electrode layer, a step of removing a residue on a surface of an insulating layer and in the vicinity thereof between the source electrode layer and the drain electrode layer is performed. The removal step can be performed by cleaning treatment using a solution or plasma treatment using a rare gas. For example, cleaning treatment using a dilute hydrofluoric acid solution, plasma treatment using argon, or the like can be suitably used.

In another embodiment of the invention disclosed in this specification, for the purpose of preventing reaction between a gas containing chlorine and an oxide semiconductor film containing indium, the oxide semiconductor film containing indium is covered with an insulating layer or a conductive film so as not to be exposed to the gas containing chlorine in an etching step using the gas containing chlorine.

Since the surface of the insulating layer and the vicinity thereof can be prevented from being contaminated by the residue, the chlorine concentration and the indium concentration on the surface of the insulating layer can be lower than or equal to 1×10¹⁹/cm³ (preferably lower than or equal to 5×10¹⁸/cm³) and lower than or equal to 2×10¹⁹/cm³ (preferably lower than or equal to 5×10¹⁸/cm³), respectively, in the semiconductor device including the staggered transistor having a bottom-gate structure. Further, the chlorine concentration in the oxide semiconductor film can be lower than or equal to 1×10¹⁹/cm³ (preferably lower than or equal to 5×10¹⁸/cm³).

Consequently, a highly reliable semiconductor device which includes a transistor including an oxide semiconductor film and having stable electric characteristics can be provided. Further, a highly reliable semiconductor device can be manufactured at a high yield, whereby high productivity is achieved.

One embodiment of the invention disclosed in this specification is a semiconductor device including a gate electrode layer over an insulating surface, a gate insulating film over the gate electrode layer, an oxide semiconductor film containing indium over the gate insulating film, an insulating layer provided on and in contact with the oxide semiconductor film so as to overlap with the gate electrode layer, and a source electrode layer and a drain electrode layer in contact with the oxide semiconductor film and the insulating layer. The chlorine concentration and the indium concentration on a surface of the insulating layer are lower than or equal to 1×10¹⁹/cm³ and lower than or equal to 2×10¹⁹/cm³, respectively.

One embodiment of the invention disclosed in this specification is a semiconductor device including a gate electrode layer over an insulating surface, a gate insulating film over the gate electrode layer, an oxide semiconductor film containing indium over the gate insulating film, an insulating layer provided on and in contact with the oxide semiconductor film so as to overlap with the gate electrode layer, and a source electrode layer and a drain electrode layer in contact with the oxide semiconductor film and the insulating layer. The chlorine concentration in the oxide semiconductor film is lower than or equal to 1×10¹⁹/cm³. The chlorine concentration and the indium concentration on a surface of the insulating layer are lower than or equal to 1×10¹⁹/cm³ and lower than or equal to 2×10¹⁹/cm³, respectively.

One embodiment of the invention disclosed in this specification is a method for manufacturing a semiconductor device comprising the steps of forming a gate electrode layer over an insulating surface, forming a gate insulating film over the gate electrode layer, forming an oxide semiconductor film containing indium over the gate insulating film, forming an insulating layer on and in contact with the oxide semiconductor film, the insulating layer overlapping with the gate electrode layer, forming a conductive film in contact with the oxide semiconductor film and the insulating layer, etching the conductive film with a gas containing chlorine to form a source electrode layer and a drain electrode layer, and performing a residue removal step on the oxide semiconductor film and the insulating layer.

One embodiment of the invention disclosed in this specification is a method for manufacturing a semiconductor device comprising the steps of forming a gate electrode layer over an insulating surface, forming a gate insulating film over the gate electrode layer, forming an oxide semiconductor film containing indium over the gate insulating film, forming an insulating layer in contact with the oxide semiconductor film to cover the oxide semiconductor film, forming openings in the insulating layer, the openings reaching the oxide semiconductor film, forming a conductive film over the insulating layer to cover inner walls of the openings, and etching the conductive film with a gas containing chlorine to form a source electrode layer and a drain electrode layer in the openings.

In addition, heat treatment by which hydrogen or moisture is eliminated from the oxide semiconductor film (dehydration or dehydrogenation treatment) may be performed on the oxide semiconductor film. In the case where a crystalline oxide semiconductor film used as the oxide semiconductor film, heat treatment for crystallization may be performed.

In addition, oxygen may be supplied to the oxide semiconductor film. The dehydration or dehydrogenation treatment may accompany elimination of oxygen which is a main constituent material of an oxide semiconductor to lead to a reduction in oxygen. An oxygen vacancy exists in a portion where oxygen is eliminated in an oxide semiconductor film, and a donor level which leads to a change in the electric characteristics of a transistor is formed owing to the oxygen vacancy.

Thus, oxygen is preferably supplied to the oxide semiconductor film after being subjected to the dehydration or dehydrogenation treatment. By supply of oxygen to the oxide semiconductor film, oxygen vacancies in the oxide semiconductor film can be repaired.

For example, an oxide insulating film including much (excessive) oxygen, which serves as a supply source of oxygen, may be provided so as to be in contact with the oxide semiconductor film, whereby oxygen can be supplied from the oxide insulating film to the oxide semiconductor film. In the above structure, heat treatment may be performed in the state where the oxide semiconductor film after being subjected to the heat treatment as the dehydration or dehydrogenation treatment and the oxide insulating film are at least partly in contact with each other to supply oxygen to the oxide semiconductor film.

Further or alternatively, oxygen (which includes at least one of an oxygen radical, an oxygen atom, and an oxygen ion) may be introduced into the oxide semiconductor film after being subjected to the dehydration or dehydrogenation treatment to supply oxygen to the oxide semiconductor film. Oxygen can be introduced by an ion implantation method, an ion doping method, a plasma immersion ion implantation method, plasma treatment, or the like.

Further, it is preferable that the oxide semiconductor film in the transistor include a region where the oxygen content is higher than that in the stoichiometric composition of the oxide semiconductor in a crystalline state. In that case, the oxygen content is preferably larger than that in the stoichiometric composition of the oxide semiconductor. Alternatively, the oxygen content is higher than that of the oxide semiconductor in a single crystal state. In some cases, oxygen may exist between lattices of the oxide semiconductor.

By removing hydrogen or moisture from the oxide semiconductor to highly purify the oxide semiconductor so as not to contain impurities as much as possible, and supplying oxygen to repair oxygen vacancies therein, the oxide semiconductor can be turned into an i-type (intrinsic) oxide semiconductor or a substantially i-type (intrinsic) oxide semiconductor. This enables the Fermi level (E_(f)) of the oxide semiconductor to be at the same level as the intrinsic Fermi level (E_(i)) thereof. Thus, by using the oxide semiconductor film for a transistor, variation in the threshold voltage Vth of the transistor due to the oxygen vacancy and a shift of the threshold voltage ΔVth can be reduced.

One embodiment of the present invention relates to a semiconductor device including a transistor or a semiconductor device including a circuit which is formed by using a transistor. For example, one embodiment of the present invention relates to a semiconductor device including a transistor in which a channel formation region is formed using an oxide semiconductor or a semiconductor device including a circuit which is formed by using such a transistor. For example, one embodiment of the present invention relates to an electronic device which includes, as a component, an LSI; a CPU; a power device mounted in a power circuit; a semiconductor integrated circuit including a memory, a thyristor, a converter, an image sensor, or the like; an electro-optical device typified by a liquid crystal display panel; or a light-emitting display device including a light-emitting element.

A highly reliable semiconductor device which includes a transistor including an oxide semiconductor is provided.

Further, a highly reliable semiconductor device is manufactured at a high yield, whereby high productivity is achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are a plan view and a cross-sectional view which illustrate one embodiment of a semiconductor device.

FIGS. 2A to 2D are cross-sectional views illustrating one embodiment of a method for manufacturing a semiconductor device.

FIGS. 3A and 3B are a plan view and a cross-sectional view which illustrate one embodiment of a semiconductor device.

FIGS. 4A to 4D illustrate one embodiment of a method for manufacturing a semiconductor device.

FIGS. 5A to 5C are plan views each illustrating one embodiment of a semiconductor device.

FIGS. 6A and 6B are a plan view and a cross-sectional view which illustrate one embodiment of a semiconductor device.

FIGS. 7A and 7B are cross-sectional views each illustrating one embodiment of a semiconductor device.

FIGS. 8A and 8B are an equivalent circuit diagram and a cross-sectional view which illustrate one embodiment of a semiconductor device.

FIGS. 9A to 9C illustrate electronic devices.

FIGS. 10A to 10C illustrate an electronic device.

FIG. 11 shows SIMS measurement results.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the invention disclosed in this specification will be described with reference to the accompanying drawings. Note that the invention disclosed in this specification is not limited to the following description, and it is easily understood by those skilled in the art that modes and details can be variously changed without departing from the spirit and the scope of the invention. Therefore, the invention disclosed in this specification is not construed as being limited to the description of the following embodiments. Note that the ordinal numbers such as “first” and “second” in this specification are used for convenience and do not denote the order of steps and the stacking order of layers. In addition, the ordinal numbers in this specification do not denote particular names which specify the present invention.

Embodiment 1

In this embodiment, one embodiment of a semiconductor device and a method for manufacturing the semiconductor device will be described with reference to FIGS. 1A and 1B and FIGS. 2A to 2D. In this embodiment, a transistor including an oxide semiconductor film will be described as an example of the semiconductor device.

The transistor may have a single-gate structure in which one channel formation region is formed, a double-gate structure in which two channel formation regions are formed, or a triple-gate structure in which three channel formation regions are formed. Alternatively, the transistor may have a dual-gate structure including two gate electrode layers positioned above and below a channel formation region with a gate insulating film provided therebetween.

A transistor 440 illustrated in FIGS. 1A and 1B is an example of a transistor which is one of bottom-gate transistors referred to as a channel-protective (channel-stop) transistor and is also referred to as an inverted-staggered transistor. FIG. 1A is a plan view, and FIG. 1B is a cross-sectional view taken along dashed-dotted line X1-Y1 in FIG. 1A.

As illustrated in FIG. 1B, which is a cross-sectional view in the channel length direction, a semiconductor device including the transistor 440 includes, over a substrate 400 having an insulating surface over which an insulating film 436 is provided, a gate electrode layer 401, a gate insulating film 402, an oxide semiconductor film 403 containing indium, an insulating layer 413, a source electrode layer 405 a, and a drain electrode layer 405 b.

The insulating layer 413 in contact with the oxide semiconductor film 403 is provided over a channel formation region of the oxide semiconductor film 403, which overlaps with the gate electrode layer 401, and functions as a channel protective film.

The cross-sectional shape of the insulating layer 413 which is over and overlaps with the channel formation region, specifically, the cross-sectional shape (e.g., taper angle and thickness) of an end portion of the insulating layer 413 is adjusted, so that electric-field concentration which might occur in the vicinity of an end portion of the drain electrode layer 405 b can be reduced and degradation of the switching characteristics of the transistor 440 can be suppressed.

Specifically, the cross-sectional shape of the insulating layer 413 which is over and overlaps with the channel formation region is set to a trapezoid or a triangle, and the taper angle of a lower end portion of the cross-sectional shape is set to 60° or less, preferably 45° or less, further preferably 30° or less. Setting the taper angle within such a range makes it possible to reduce the electric-field concentration which might occur in the vicinity of the end portion of the drain electrode layer 405 b when a high gate voltage is applied to the gate electrode layer 401.

In this embodiment, the end portion of the insulating layer 413 is tapered on the outer side than a central area D.

The thickness of the insulating layer 413 which is over and overlaps with the channel formation region is less than or equal to 0.3 μm, preferably greater than or equal to 5 nm and less than or equal to 0.1 μm. Setting the thickness within such a range makes it possible to reduce the peak of electric-field intensity, or distribute the electric-field concentration so that the electric-field is concentrated in plural portions, consequently reducing the electric-field concentration which might occur in the vicinity of the end portion of the drain electrode layer 405 b.

An oxide semiconductor used for the oxide semiconductor film 403 contains at least indium (In). In particular, In and zinc (Zn) are preferably contained. In addition, as a stabilizer for reducing variation in electric characteristics of a transistor formed using the oxide, gallium (Ga) is preferably contained in addition to In and Zn. Tin (Sn) is preferably contained as a stabilizer. Hafnium (Hf) is preferably contained as a stabilizer. Aluminum (Al) is preferably contained as a stabilizer. Zirconium (Zr) is preferably contained as a stabilizer.

As another stabilizer, one or plural kinds of lanthanoid such as lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), or lutetium (Lu) may be contained.

As the oxide semiconductor, for example, any of the following can be used: indium oxide; tin oxide; zinc oxide; a two-component metal oxide such as an In—Zn-based oxide, an In—Mg-based oxide, or an In—Ga-based oxide; a three-component metal oxide such as an In—Ga—Zn-based oxide (also referred to as IGZO), an In—Al—Zn-based oxide, an In—Sn—Zn-based oxide, an In—Hf—Zn-based oxide, an In—La—Zn-based oxide, an In—Ce—Zn-based oxide, an In—Pr—Zn-based oxide, an In—Nd—Zn-based oxide, an In—Sm—Zn-based oxide, an In—Eu—Zn-based oxide, an In—Gd—Zn-based oxide, an In—Tb—Zn-based oxide, an In—Dy—Zn-based oxide, an In—Ho—Zn-based oxide, an In—Er—Zn-based oxide, an In—Tm—Zn-based oxide, an In—Yb—Zn-based oxide, or an In—Lu—Zn-based oxide; and a four-component metal oxide such as an In—Sn—Ga—Zn-based oxide, an In—Hf—Ga—Zn-based oxide, an In—Al—Ga—Zn-based oxide, an In—Sn—Al—Zn-based oxide, an In—Sn—Hf—Zn-based oxide, or an In—Hf—Al—Zn-based oxide.

Note that here, for example, an “In—Ga—Zn-based oxide” means an oxide containing In, Ga, and Zn as its main component and there is no particular limitation on the ratio of In:Ga:Zn. The In—Ga—Zn-based oxide may contain a metal element other than the In, Ga, and Zn.

A material represented by InMO₃(ZnO)_(m) (m>0, where m is not an integer) may be used as the oxide semiconductor. Note that M represents one or more metal elements selected from Ga, Fe, Mn, and Co. Alternatively, a material represented by In₂SnO₅(ZnO)_(n) (n>0, where n is an integer) may be used as the oxide semiconductor.

For example, an In—Ga—Zn-based oxide with an atomic ratio of In:Ga:Zn=1:1:1 (=⅓:⅓:⅓), In:Ga:Zn=2:2:1 (=⅖:⅖:⅕), or In:Ga:Zn=3:1:2 (=½:⅙:⅓), or an oxide with an atomic ratio close to the above atomic ratios can be used. Alternatively, an In—Sn—Zn-based oxide with an atomic ratio of In:Sn:Zn=1:1:1 (=⅓:⅓:⅓), In:Sn:Zn=2:1:3 (=⅓:⅙:½), or In:Sn:Zn=2:1:5 (=¼:⅛:⅝), or an oxide with an atomic ratio close to the above atomic ratios may be used.

However, without limitation to the materials given above, a material with an appropriate composition may be used as the oxide semiconductor containing indium depending on needed semiconductor characteristics (e.g., mobility, threshold voltage, and variation). In order to obtain the needed semiconductor characteristics, it is preferable that the carrier density, the impurity concentration, the defect density, the atomic ratio between a metal element and oxygen, the interatomic distance, the density, and the like be set to appropriate values.

For example, high mobility can be obtained relatively easily in the case of using an In—Sn—Zn-based oxide. However, mobility can be increased by reducing the defect density in a bulk also in the case of using an In—Ga—Zn-based oxide.

For example, in the case where the composition of an oxide containing In, Ga, and Zn at the atomic ratio, In:Ga:Zn=a:b:c (a+b+c=1), is in the neighborhood of the composition of an oxide containing In, Ga, and Zn at the atomic ratio, In:Ga:Zn=A:B:C (A+B+C=1), a, b, and c satisfy the following relation: (a−A)²+(b−B)²+(c−C)²≦r², and r may be 0.05, for example. The same applies to other oxides.

The oxide semiconductor film 403 is in a single crystal state, a polycrystalline (also referred to as polycrystal) state, an amorphous state, or the like.

The oxide semiconductor film is preferably a c-axis aligned crystalline oxide semiconductor (CAAC-OS) film.

The CAAC-OS film is not completely single crystal nor completely amorphous. The CAAC-OS film is an oxide semiconductor film with a crystal-amorphous mixed phase structure where crystal parts are included in an amorphous phase. Note that in most cases, the crystal part fits inside a cube whose one side is less than 100 nm. From an observation image obtained with a transmission electron microscope (TEM), a boundary between an amorphous part and a crystal part in the CAAC-OS film is not clear. Further, with the TEM, a grain boundary in the CAAC-OS film is not found. Thus, in the CAAC-OS film, a reduction in electron mobility, due to the grain boundary, is suppressed.

In each of the crystal parts included in the CAAC-OS film, a c-axis is aligned in a direction parallel to a normal vector of a surface where the CAAC-OS film is formed or a normal vector of a surface of the CAAC-OS film, triangular or hexagonal atomic arrangement which is seen from the direction perpendicular to the a-b plane is formed, and metal atoms are arranged in a layered manner or metal atoms and oxygen atoms are arranged in a layered manner when seen from the direction perpendicular to the c-axis. Note that, among crystal parts, the directions of the a-axis and the b-axis of one crystal part may be different from those of another crystal part. In this specification, a simple term “perpendicular” includes a range from 85° to 95°. In addition, a simple term “parallel” includes a range from −5° to 5°.

In the CAAC-OS film, distribution of crystal parts is not necessarily uniform. For example, in the formation process of the CAAC-OS film, in the case where crystal growth occurs from a surface side of the oxide semiconductor film, the proportion of crystal parts in the vicinity of the surface of the oxide semiconductor film is higher than that in the vicinity of the surface where the oxide semiconductor film is formed in some cases. Further, when an impurity is added to the CAAC-OS film, the crystal part in a region to which the impurity is added becomes amorphous in some cases.

Since the c-axes of the crystal parts included in the CAAC-OS film are aligned in the direction parallel to a normal vector of a surface where the CAAC-OS film is formed or a normal vector of a surface of the CAAC-OS film, the directions of the c-axes may be different from each other depending on the shape of the CAAC-OS film (the cross-sectional shape of the surface where the CAAC-OS film is formed or the cross-sectional shape of the surface of the CAAC-OS film). Note that when the CAAC-OS film is formed, the direction of c-axis of the crystal part is the direction parallel to a normal vector of the surface where the CAAC-OS film is formed or a normal vector of the surface of the CAAC-OS film. The crystal part is formed by film formation or by performing treatment for crystallization such as heat treatment after film formation.

With use of the CAAC-OS film in a transistor, change in electric characteristics of the transistor due to irradiation with visible light or ultraviolet light can be reduced. Thus, the transistor has high reliability.

Note that part of oxygen included in the oxide semiconductor film may be substituted with nitrogen.

In an oxide semiconductor having a crystal part such as the CAAC-OS, defects in the bulk can be further reduced and when the surface flatness of the oxide semiconductor is improved, mobility higher than that of an oxide semiconductor in an amorphous state can be obtained. In order to improve the surface flatness, the oxide semiconductor is preferably formed over a flat surface. Specifically, the oxide semiconductor may be formed over a surface with the average surface roughness (R_(a)) of less than or equal to 1 nm, preferably less than or equal to 0.3 nm, further preferably less than or equal to 0.1 nm.

Note that R_(a) is obtained by expanding arithmetic mean surface roughness, which is defined by JIS B 0601: 2001 (ISO4287: 1997), into three dimensions so as to be applied to a curved surface. R_(a) can be expressed as an “average value of the absolute values of deviations from a reference surface to a specific surface” and is defined by the following formula.

$\begin{matrix} {{Ra} = {\frac{1}{S_{0}}{\int_{y_{1}}^{y_{2}}{\int_{x_{1}}^{x_{2}}{{{{f\left( {x,y} \right)} - Z_{0}}}{\mathbb{d}x}{\mathbb{d}y}}}}}} & \left\lbrack {{FORMULA}\mspace{14mu} 1} \right\rbrack \end{matrix}$

Here, the specific surface is a surface which is a target of roughness measurement, and is a quadrilateral region which is specified by four points represented by the coordinates (x₁, y₁, f(x₁, y₁)), (x₁, y₂, f(x₁, y₂)), (x₂, y₁, f(x₂, y₁)), and (x₂, y₂, f(x₂, y₂)). S₀ represents the area of a rectangle which is obtained by projecting the specific surface on the xy plane, and Z₀ represents the height of the reference surface (the average height of the specific surface). R_(a) can be measured using an atomic force microscope (AFM).

Note that since the transistor 440 described in this embodiment is a bottom-gate transistor, the substrate 400, the gate electrode layer 401, and the gate insulating film 402 are located below the oxide semiconductor film. Accordingly, planarization treatment such as CMP treatment may be performed after the formation of the gate electrode layer 401 and the gate insulating film 402 to obtain the above flat surface. Other than planarizing the entire surface of the desired portion, it is also possible to sufficiently increase the distance between a side surface of the gate electrode layer 401 and the lower end portion of the insulating layer 413 so that at least a region which is to be the channel formation region can have a surface as flat as the above flat surface. Since the transistor 440 is a channel-protective transistor, the size (L/W) of the channel formation region is determined by the size of the insulating layer 413.

The oxide semiconductor film 403 has a thickness greater than or equal to 1 nm and less than or equal to 30 nm (preferably greater than or equal to 5 nm and less than or equal to 10 nm) and can be formed by a sputtering method, a molecular beam epitaxy (MBE) method, a CVD method, a pulse laser deposition method, an atomic layer deposition (ALD) method, or the like as appropriate. The oxide semiconductor film 403 may be formed with a sputtering apparatus which performs deposition in the state where top surfaces of a plurality of substrates are substantially perpendicular to a top surface of a sputtering target.

For example, the CAAC-OS film is formed by a sputtering method with a polycrystalline oxide semiconductor sputtering target. When ions collide with the sputtering target, a crystal region included in the sputtering target may be separated from the target along an a-b plane; in other words, a sputtered particle having a plane parallel to an a-b plane (flat-plate-like sputtered particle or pellet-like sputtered particle) may flake off from the sputtering target. In that case, the flat-plate-like sputtered particles reach a substrate while maintaining their crystal state, whereby the CAAC-OS film can be formed.

For the deposition of the CAAC-OS film, the following conditions are preferably used.

By reducing the amount of impurities entering the CAAC-OS film during the deposition, the crystal state can be prevented from being broken by the impurities. For example, the concentration of impurities (e.g., hydrogen, water, carbon dioxide, or nitrogen) which exist in the deposition chamber may be reduced. Furthermore, the concentration of impurities in a deposition gas may be reduced. Specifically, a deposition gas whose dew point is −80° C. or lower, preferably −100° C. or lower is used.

By increasing the substrate heating temperature during the deposition, migration of a sputtered particle is likely to occur after the sputtered particle reaches a substrate surface. Specifically, the substrate heating temperature during the deposition is higher than or equal to 100° C. and lower than or equal to 740° C., preferably higher than or equal to 200° C. and lower than or equal to 500° C. By increasing the substrate heating temperature during the deposition, when the flat-plate-like sputtered particle reaches the substrate, migration occurs on the substrate surface, so that a flat plane of the flat-plate-like sputtered particle is attached to the substrate.

Furthermore, it is preferable that the proportion of oxygen in the deposition gas be increased and the power be optimized in order to reduce plasma damage at the deposition. The proportion of oxygen in the deposition gas is 30 vol % or higher, preferably 100 vol %.

As an example of the sputtering target, an In—Ga—Zn—O compound target is described below.

The In—Ga—Zn—O compound target, which is polycrystalline, is made by mixing InO_(x) powder, GaO_(y) powder, and ZnO_(z) powder in a predetermined molar ratio, applying pressure, and performing heat treatment at a temperature higher than or equal to 1000° C. and lower than or equal to 1500° C. Note that X, Y, and Z are each a given positive number. Here, the predetermined molar ratio of InO_(x) powder to GaO_(y) powder and ZnO_(z) powder is, for example, 2:2:1, 8:4:3, 3:1:1, 1:1:1, 4:2:3, or 3:1:2. The kinds of powder and the molar ratio for mixing powder may be determined as appropriate depending on the desired sputtering target.

FIGS. 2A to 2D illustrate an example of a method for manufacturing the semiconductor device including the transistor 440.

First, the insulating film 436 is formed over the substrate 400 having an insulating surface.

There is no particular limitation on a substrate that can be used as the substrate 400 having an insulating surface as long as it has heat resistance enough to withstand heat treatment performed later. For example, a glass substrate of barium borosilicate glass, aluminoborosilicate glass, or the like, a ceramic substrate, a quartz substrate, or a sapphire substrate can be used. A single crystal semiconductor substrate or a polycrystalline semiconductor substrate of silicon, silicon carbide, or the like; a compound semiconductor substrate of silicon germanium or the like; an SOI substrate; or the like can be used as the substrate 400, or the substrate provided with a semiconductor element can be used as the substrate 400.

The semiconductor device may be manufactured using a flexible substrate as the substrate 400. To manufacture a flexible semiconductor device, the transistor 440 including the oxide semiconductor film 403 may be directly formed over a flexible substrate; or alternatively, the transistor 440 including the oxide semiconductor film 403 may be formed over a manufacturing substrate, and then may be separated and transferred to a flexible substrate. Note that in order to separate the transistor 440 from the manufacturing substrate and transfer it to the flexible substrate, a separation layer may be provided between the manufacturing substrate and the transistor 440 including the oxide semiconductor film.

The insulating film 436 can be formed by a plasma CVD method, a sputtering method, or the like using an oxide insulating material such as silicon oxide, silicon oxynitride, aluminum oxide, aluminum oxynitride, hafnium oxide, or gallium oxide; a nitride insulating material such as silicon nitride, silicon nitride oxide, aluminum nitride, or aluminum nitride oxide; or a mixed material of any of these materials.

The insulating film 436 may have either a single-layer structure or a stacked-layer structure.

In this embodiment, a stack of a 100-nm-thick silicon nitride film and a 150-nm-thick silicon oxide film, which are formed by a plasma CVD method, is used as the insulating film 436.

Next, a conductive film is formed over the insulating film 436 and etched, whereby the gate electrode layer 401 is formed.

The gate electrode layer 401 can be formed using a metal material such as molybdenum, titanium, tantalum, tungsten, aluminum, copper, chromium, neodymium, or scandium or an alloy material which contains any of these materials as its main component. A semiconductor film which is doped with an impurity element such as phosphorus and is typified by a polycrystalline silicon film, or a silicide film of nickel silicide or the like can also be used as the gate electrode layer 401. The gate electrode layer 401 has either a single-layer structure or a stacked-layer structure.

The gate electrode layer 401 can also be formed using a conductive material such as indium oxide-tin oxide, indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, indium oxide-zinc oxide, or indium tin oxide to which silicon oxide is added. It is also possible that the gate electrode layer 401 has a stacked-layer structure of the above conductive material and the above metal material.

As one layer of the gate electrode layer 401 which is in contact with the gate insulating film 402, a metal oxide containing nitrogen, specifically, an In—Ga—Zn—O film containing nitrogen, an In—Sn—O film containing nitrogen, an In—Ga—O film containing nitrogen, an In—Zn—O film containing nitrogen, a Sn—O film containing nitrogen, an In—O film containing nitrogen, or a metal nitride (e.g., InN or SnN) film can be used. These films each have a work function of 5 eV or higher, preferably 5.5 eV or higher, which enables the threshold voltage which is one of the electrical characteristics of the transistor to take a positive value when used as the gate electrode layer, so that a switching element of so-called normally-off type can be achieved.

In this embodiment, a 100-nm-thick tungsten film is formed by a sputtering method.

Next, the gate insulating film 402 is formed over the gate electrode layer 401.

To improve the coverage with the gate insulating film 402, planarization treatment may be performed on a surface of the gate electrode layer 401. It is preferable that the flatness of the surface of the gate electrode layer 401 be good particularly when the thickness of the gate insulating film 402 is small.

The gate insulating film 402 has a thickness greater than or equal to 1 nm and less than or equal to 20 nm and can be formed by a sputtering method, an MBE method, a CVD method, a pulsed laser deposition method, an ALD method, or the like as appropriate. The gate insulating film 402 may also be formed with a sputtering apparatus which performs deposition in the state where top surfaces of a plurality of substrates are substantially perpendicular to a top surface of a sputtering target.

The gate insulating film 402 can be formed using a silicon oxide film, a gallium oxide film, an aluminum oxide film, a silicon nitride film, a silicon oxynitride film, an aluminum oxynitride film, or a silicon nitride oxide film.

The gate insulating film 402 can be formed using a high-k material such as hafnium oxide, yttrium oxide, hafnium silicate (HfSi_(x)O_(y) (x>0, y>0)), hafnium silicate to which nitrogen is added (HfSiO_(x)N_(y) (x>0, y>0)), hafnium aluminate (HfAl_(x)O_(y) (x>0, y>0)), or lanthanum oxide, whereby gate leakage current can be reduced. Further, the gate insulating film 402 may have either a single-layer structure or a stacked-layer structure.

It is preferable that the gate insulating film 402 contain oxygen in a portion in contact with the oxide semiconductor film 403. In particular, the gate insulating film 402 preferably contains a large amount of oxygen which exceeds at least the amount of oxygen in the stoichiometric composition in the film (bulk). For example, in the case where a silicon oxide film is used as the gate insulating film 402, the composition formula is SiO_(2+α) (α>0).

When the gate insulating film 402 containing much (excess) oxygen, which serves as an oxygen supply source, is provided so as to be in contact with the oxide semiconductor film 403, oxygen can be supplied from the gate insulating film 402 to the oxide semiconductor film 403. Heat treatment may be performed in the state where the oxide semiconductor film 403 and the gate insulating film 402 are at least partly in contact with each other to supply oxygen to the oxide semiconductor film 403.

By supply of oxygen to the oxide semiconductor film 403, oxygen vacancies in the film can be repaired. Further, the gate insulating film 402 is preferably formed in consideration of the size of a transistor to be formed and the step coverage with the gate insulating film 402.

In this embodiment, a 200-nm-thick silicon oxynitride film is formed by a high-density plasma CVD method.

Next, the oxide semiconductor film 403 is formed over the gate insulating film 402.

In order to prevent the oxide semiconductor film 403 from containing hydrogen or water as much as possible in the step of forming the oxide semiconductor film 403, it is preferable to heat the substrate provided with the gate insulating film 402 in a preheating chamber in a sputtering apparatus as a pretreatment for formation of the oxide semiconductor film 403 so that impurities such as hydrogen and moisture adsorbed on the substrate and the gate insulating film 402 are eliminated and removed. As an exhaustion unit provided in the preheating chamber, a cryopump is preferable.

Planarization treatment may be performed on a region of the gate insulating film 402 which is in contact with the oxide semiconductor film 403. The planarization treatment may be, but not particularly limited to, a polishing treatment (such as chemical mechanical polishing (CMP)), a dry etching treatment, or a plasma treatment.

As plasma treatment, reverse sputtering in which an argon gas is introduced and plasma is generated can be performed, for example. The reverse sputtering is a method in which voltage is applied to a substrate side with use of an RF power source in an argon atmosphere and plasma is generated in the vicinity of the substrate so that a substrate surface is modified. Note that instead of an argon atmosphere, a nitrogen atmosphere, a helium atmosphere, an oxygen atmosphere, or the like may be used. The reverse sputtering can remove particle substances (also referred to as particles or dust) attached to a surface of the gate insulating film 402.

As the planarization treatment, polishing treatment, dry etching treatment, or plasma treatment may be performed plural times, or these treatments may be performed in combination. In the case where the treatments are combined, the order of steps is not particularly limited and may be set as appropriate depending on the roughness of the surface of the gate insulating film 402.

The oxide semiconductor film 403 is preferably formed under a condition such that much oxygen is contained (for example, by a sputtering method in an atmosphere where the proportion of oxygen is 100%) so as to be a film containing much oxygen (preferably having a region where the oxygen content is in excess of that in the stoichiometric composition of the oxide semiconductor in a crystalline state).

In this embodiment, a 35-nm-thick In—Ga—Zn-based oxide film (IGZO film) is formed as the oxide semiconductor film 403 by a sputtering method using a sputtering apparatus including an AC power supply device. In this embodiment, an In—Ga—Zn-based oxide target having an atomic ratio of In:Ga:Zn=1:1:1 (=⅓:⅓:⅓) is used. The deposition conditions are as follows: the atmosphere is oxygen and argon (the proportion of the oxygen flow rate is 50%), the pressure is 0.6 Pa, the electric power is 5 kW, and the substrate temperature is 170° C. The deposition rate under these deposition conditions is 16 nm/min.

It is preferable that a high-purity gas from which an impurity such as hydrogen, water, a hydroxyl group, and hydride is removed be used as a sputtering gas for the formation of the oxide semiconductor film 403.

The substrate is held in a deposition chamber kept under reduced pressure. Then, a sputtering gas from which hydrogen and moisture have been removed is introduced into the deposition chamber while moisture remaining therein is removed, and the oxide semiconductor film 403 is formed over the substrate 400 with the use of the above target. In order to remove moisture remaining in the deposition chamber, an entrapment vacuum pump such as a cryopump, an ion pump, or a titanium sublimation pump is preferably used. As an exhaustion unit, a turbo molecular pump to which a cold trap is added may be used. In the deposition chamber which is evacuated with the cryopump, for example, a hydrogen atom, a compound containing a hydrogen atom, such as water (H₂O), (further preferably, also a compound containing a carbon atom), and the like are removed, whereby the concentration of impurities in the oxide semiconductor film 403 formed in the deposition chamber can be reduced.

It is preferable to form the gate insulating film 402 and the oxide semiconductor film 403 in succession so as not to expose the gate insulating film 402 to the air. Forming the gate insulating film 402 and the oxide semiconductor film 403 in succession so as not to expose the gate insulating film 402 to the air can prevent impurities such as hydrogen and moisture from being adsorbed onto the surface of the gate insulating film 402.

The oxide semiconductor film 403 can be formed by processing an oxide semiconductor film into an island shape by a photolithography process.

A resist mask for forming the island-shaped oxide semiconductor film 403 may be formed by an inkjet method. Formation of the resist mask by an inkjet method needs no photomask; thus, manufacturing cost can be reduced.

Note that the etching of the oxide semiconductor film may be dry etching, wet etching, or both dry etching and wet etching. As an etchant used for wet etching of the oxide semiconductor film, for example, a mixed solution of phosphoric acid, acetic acid, and nitric acid, or the like can be used. Alternatively, ITO-07N (produced by KANTO CHEMICAL CO., INC.) may be used. It is also possible to employ dry etching by an inductively coupled plasma (ICP) etching method.

Further, heat treatment may be performed on the oxide semiconductor film 403 in order to remove excess hydrogen (including water and a hydroxyl group) (to perform dehydration or dehydrogenation treatment). The temperature of the heat treatment is higher than or equal to 300° C. and lower than or equal to 700° C., or lower than the strain point of the substrate. The heat treatment can be performed under reduced pressure, a nitrogen atmosphere, or the like.

In this embodiment, the substrate is introduced into an electric furnace which is one of heat treatment apparatuses, and the oxide semiconductor film 403 is subjected to heat treatment at 450° C. in a nitrogen atmosphere for one hour and further to heat treatment at 450° C. in an atmosphere containing nitrogen and oxygen for one hour.

Note that a heat treatment apparatus used is not limited to an electric furnace, and a device for heating an object to be processed by heat conduction or heat radiation from a heating element such as a resistance heating element may be alternatively used. For example, a rapid thermal annealing (RTA) apparatus such as a gas rapid thermal annealing (GRTA) apparatus or a lamp rapid thermal annealing (LRTA) apparatus can be used. An LRTA apparatus is an apparatus for heating an object to be processed by radiation of light (an electromagnetic wave) emitted from a lamp such as a halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arc lamp, a high pressure sodium lamp, or a high pressure mercury lamp. A GRTA apparatus is an apparatus for heat treatment using a high-temperature gas. As the high-temperature gas, an inert gas which does not react with an object to be processed by heat treatment, such as nitrogen or a rare gas like argon, is used.

For example, as the heat treatment, GRTA may be performed as follows. The substrate is put in an inert gas heated at high temperature of 650° C. to 700° C., is heated for several minutes, and is taken out of the inert gas.

Note that in the heat treatment, it is preferable that water, hydrogen, and the like be not contained in nitrogen or a rare gas such as helium, neon, or argon. The purity of nitrogen or the rare gas such as helium, neon, or argon which is introduced into the heat treatment apparatus is set to preferably 6N (99.9999%) or higher, further preferably 7N (99.99999%) or higher (that is, the impurity concentration is preferably 1 ppm or lower, further preferably 0.1 ppm or lower).

In addition, after the oxide semiconductor film 403 is heated by the heat treatment, a high-purity oxygen gas, a high-purity N₂O gas, or ultra dry air (the moisture amount is less than or equal to 20 ppm (−55° C. by conversion into a dew point), preferably less than or equal to 1 ppm, further preferably less than or equal to 10 ppb according to the measurement with a dew point meter of a cavity ring down laser spectroscopy (CRDS) system) may be introduced into the same furnace. It is preferable that water, hydrogen, or the like be not contained in the oxygen gas or the N₂O gas. The purity of the oxygen gas or the N₂O gas which is introduced into the heat treatment apparatus is preferably 6N or higher, further preferably 7N or higher (i.e., the impurity concentration of the oxygen gas or the N₂O gas is preferably 1 ppm or lower, further preferably 0.1 ppm or lower). The oxygen gas or the N₂O gas acts to supply oxygen that is a main constituent material of the oxide semiconductor and that is reduced by the step for removing an impurity for dehydration or dehydrogenation, so that the oxide semiconductor film 403 can be a high-purified, i-type (intrinsic) oxide semiconductor film.

Note that the timing of the heat treatment for dehydration or dehydrogenation may be after formation of the oxide semiconductor film or after formation of the island-shaped oxide semiconductor film 403.

The heat treatment for dehydration or dehydrogenation may be performed plural times and may also serve as another heat treatment.

When the heat treatment for dehydration or dehydrogenation is performed in the state where the gate insulating film 402 is covered with the oxide semiconductor film which has not been processed into the island-shaped oxide semiconductor film 403, oxygen contained in the gate insulating film 402 can be prevented from being released by the heat treatment, which is preferable.

Further or alternatively, oxygen (which includes at least one of an oxygen radical, an oxygen atom, and an oxygen ion) may be introduced into the oxide semiconductor film 403 after being subjected to the dehydration or dehydrogenation treatment to supply oxygen to the oxide semiconductor film 403.

The dehydration or dehydrogenation treatment may accompany elimination of oxygen which is a main constituent material of an oxide semiconductor to lead to a reduction in oxygen. An oxygen vacancy exists in a portion where oxygen is eliminated in an oxide semiconductor film, and a donor level which leads to a change in the electric characteristics of a transistor is formed owing to the oxygen vacancy.

Oxygen which is introduced into the dehydrated or dehydrogenated oxide semiconductor film 403 to supply oxygen to the film can highly purify the oxide semiconductor film 403 and make the film an i-type (intrinsic). Variation in electric characteristics of a transistor having a highly-purified and i-type (intrinsic) oxide semiconductor film 403 is suppressed, and the transistor is electrically stable.

Oxygen can be introduced by an ion implantation method, an ion doping method, a plasma immersion ion implantation method, plasma treatment, or the like.

In the case where oxygen is introduced into the oxide semiconductor film 403, oxygen may be directly introduced into the oxide semiconductor film 403, or may be introduced into the oxide semiconductor film 403 through another film such as the gate insulating film 402. An ion implantation method, an ion doping method, a plasma immersion ion implantation method, or the like may be employed for the introduction of oxygen through another film, whereas plasma treatment or the like can also be employed for the introduction of oxygen directly into the oxide semiconductor film 403 in the state where the oxide semiconductor film 403 is exposed.

The introduction of oxygen to the oxide semiconductor film 403 is preferably performed after dehydration or dehydrogenation treatment is performed thereon, but the timing is not limited thereto. Further, oxygen may be introduced plural times into the oxide semiconductor film 403 subjected to the dehydration or dehydrogenation treatment.

Next, the insulating layer 413 is formed over the channel formation region of the oxide semiconductor film 403, which overlaps with the gate electrode layer 401 (see FIG. 2A).

The insulating layer 413 can be formed by etching an insulating film which is formed by a plasma CVD method or a sputtering method. As the insulating layer 413, a single layer or a stack of one or more inorganic insulating films, typical examples of which are a silicon oxide film, a silicon oxynitride film, an aluminum oxide film, an aluminum oxynitride film, a hafnium oxide film, a gallium oxide film, a silicon nitride film, an aluminum nitride film, a silicon nitride oxide film, and an aluminum nitride oxide film, can be used.

When the insulating layer 413 in contact with the oxide semiconductor film 403 (or a film in contact with the oxide semiconductor film 403 in the case where the insulating layer 413 has a stacked-layer structure) contains much oxygen, the insulating layer 413 (or the film in contact with the oxide semiconductor film 403) can favorably function as a supply source which supplies oxygen to the oxide semiconductor film 403.

In this embodiment, a 200-nm-thick silicon oxide film is formed by a sputtering method as the insulating layer 413. The silicon oxide film is selectively etched to form the insulating layer 413 having a trapezoidal or triangular cross-sectional shape whose taper angle at the lower end portion is 60° or less, preferably 45° or less, further preferably 30° or less. The planar shape of the insulating layer 413 is a rectangle. In this embodiment, a resist mask is formed over the silicon oxide film through a photolithography process, and selective etching is performed to form a taper with an angle of about 30° at the lower end portion of the insulating layer 413.

Heat treatment may be performed after the insulating layer 413 is formed. In this embodiment, the heat treatment is performed at 300° C. for one hour in a nitrogen atmosphere.

Next, a conductive film 445 for forming a source electrode layer and a drain electrode layer (including a wiring formed of the same layer as the source electrode layer and the drain electrode layer) is formed over the gate electrode layer 401, the gate insulating film 402, the oxide semiconductor film 403, and the insulating layer 413 (see FIG. 2B).

The conductive film 445 is formed using a material that can withstand heat treatment in a later step. As the conductive film 445 used for forming the source electrode layer and the drain electrode layer, it is possible to use, for example, a metal film containing an element selected from Al, Cr, Cu, Ta, Ti, Mo, and W, a metal nitride film containing any of these elements as its component (a titanium nitride film, a molybdenum nitride film, or a tungsten nitride film), or the like. Alternatively, a film of a high-melting-point metal such as Ti, Mo, or W or a metal nitride film thereof (a titanium nitride film, a molybdenum nitride film, or a tungsten nitride film) may be formed over or/and below a metal film such as an Al film or a Cu film. Further alternatively, the conductive film 445 used for the source electrode layer and the drain electrode layer may be formed using a conductive metal oxide. As the conductive metal oxide, indium oxide (In₂O₃), tin oxide (SnO₂), zinc oxide (ZnO), indium oxide-tin oxide (In₂O₃—SnO₂), indium oxide-zinc oxide (In₂O₃—ZnO), or any of these metal oxide materials in which silicon oxide is contained can be used.

Through a photolithography process, resist masks 448 a and 448 b are formed over the conductive film 445 and selective etching is performed, so that the source electrode layer 405 a and the drain electrode layer 405 b are formed (see FIG. 2C). After formation of the source electrode layer 405 a and the drain electrode layer 405 b, the resist masks are removed. Consequently, an end portion of the drain electrode layer 405 b and an end portion of the source electrode layer 405 a are each located on a top surface or a side surface of the insulating layer 413.

A gas 447 containing chlorine is used for the etching of the conductive film 445. As the gas 447 containing chlorine, for example, a gas containing chlorine (Cl₂), boron trichloride (BCl₃), silicon tetrachloride (SiCl₄), carbon tetrachloride (CCl₄), or the like can be used.

As the etching method, a parallel-plate reactive ion etching (RIE) method or an inductively coupled plasma (ICP) etching method can be used. In order to etch the film into desired shapes, the etching condition (the amount of electric power applied to a coil-shaped electrode, the amount of electric power applied to an electrode on a substrate side, the temperature of the electrode on the substrate side, or the like) is adjusted as appropriate.

In this embodiment, a stack of a 100-nm-thick titanium film, a 400-nm-thick aluminum film, and a 100-nm-thick titanium film, which are formed by a sputtering method, is used as the conductive film 445. As the etching of the conductive film 445, the stack of the titanium film, the aluminum film, and the titanium film is etched by dry etching, whereby the source electrode layer 405 a and the drain electrode layer 405 b are formed.

In this embodiment, the upper titanium film and the aluminum film are etched under first etching conditions and then the lower titanium film is etched under second etching conditions. The first etching conditions are as follows: an etching gas (BCl₃:Cl₂=750 sccm:150 sccm) is used, the bias power is 1500 W, the ICP power is 0 W, and the pressure is 2.0 Pa. The second etching conditions are as follows: an etching gas (BCl₃:Cl₂=700 sccm:100 sccm) is used, the bias power is 750 W, the ICP power is 0 W, and the pressure is 2.0 Pa.

As described above, the gas 447 containing chlorine is used in the etching step for forming the source electrode layer 405 a and the drain electrode layer 405 b. However, when the oxide semiconductor film 403 containing indium is exposed to the gas 447 containing chlorine, reaction occurs therebetween to generate residues. Further, in the case where the residues are scattered, the residues exist on the surface of the insulating layer 413 and in the vicinity thereof between the source electrode layer 405 a and the drain electrode layer 405 b, as well as on the oxide semiconductor film 403. The residues on the surface of the insulating layer 413 and in the vicinity thereof between the source electrode layer 405 a and the drain electrode layer 405 b cause degradation of electric characteristics of the transistor 440, such as an increase in leakage current. Further, chlorine contained in the gas (in some cases, also an element other than chlorine contained in the gas) might enter or be attached to the oxide semiconductor film 403 to adversely affect transistor characteristics.

The residue includes, for example, a compound containing indium and chlorine. The residue may further include a metal element other than indium contained in the oxide semiconductor film (e.g., gallium or zinc) or an element other than chlorine used in the gas containing chlorine (e.g., boron).

Therefore, after the formation of the source electrode layer 405 a and the drain electrode layer 405 b, a step of removing the residues on the surface of the insulating layer 413 and in the vicinity thereof between the source electrode layer 405 a and the drain electrode layer 405 b is performed. The step of removing the residues can be performed by cleaning treatment using a solution or plasma treatment using a rare gas. For example, cleaning treatment using a dilute hydrofluoric acid solution, plasma treatment using argon, or the like can be suitably used. Note that the step of removing the residues also has an effect of removing chlorine which has entered or been attached to the oxide semiconductor film 403.

As described above, performing the step of removing the residues can prevent the surface of the insulating layer 413 and the vicinity thereof and the oxide semiconductor film from being contaminated by the residues, so that the chlorine concentration and the indium concentration on the surface of the insulating layer 413 can be lower than or equal to 1×10¹⁹/cm³ (preferably lower than or equal to 5×10¹⁸/cm³) and lower than or equal to 2×10¹⁹/cm³ (preferably lower than or equal to 5×10¹⁸/cm³), respectively, in the semiconductor device including the transistor 440, which is an inverted-staggered transistor having a bottom-gate structure. Further, the chlorine concentration in the oxide semiconductor film 403 can be lower than or equal to 1×10¹⁹/cm³ (preferably lower than or equal to 5×10¹⁸/cm³).

Through the above process, the transistor 440 of this embodiment can be manufactured (see FIG. 2D). Note that an insulating film serving as a protective insulating film may be formed over the source electrode layer 405 a and the drain electrode layer 405 b.

The protective insulating film can be formed using a material and a method similar to those of the insulating layer 413. For example, a 400-nm-thick silicon oxynitride film is formed by a CVD method. Heat treatment may be performed after formation of the protective insulating film. For example, heat treatment is performed at 300° C. for one hour in a nitrogen atmosphere.

Alternatively, a dense inorganic insulating film may be provided as the protective insulating film. For example, as the protective insulating film, an aluminum oxide film is formed by a sputtering method. Providing an aluminum oxide film having high density (film density: 3.2 g/cm³ or higher, preferably 3.6 g/cm³ or higher) can stabilize the electric characteristics of the transistor 440. The film density can be measured by Rutherford backscattering spectrometry (RBS) or X-ray reflection (XRR).

An aluminum oxide film which can be used as the protective insulating film provided over the transistor 440 has a high shielding effect (blocking effect) of preventing penetration of both oxygen and an impurity such as hydrogen or moisture.

Therefore, in and after the manufacturing process, the aluminum oxide film functions as a protective film for preventing entry of an impurity such as hydrogen or moisture, which causes a change of electric characteristics, into the oxide semiconductor film 403 and release of oxygen, which is a main constituent material of the oxide semiconductor, from the oxide semiconductor film 403. This is why the aluminum oxide film is preferred to be used.

In addition, a planarization insulating film may be formed in order to reduce surface unevenness due to the transistor 440. As the planarization insulating film, an organic material such as a polyimide-, acrylic-, or benzocyclobutene-based resin can be used. Other than such organic materials, it is also possible to use a low-dielectric constant material (a low-k material) or the like. Note that the planarization insulating film may be formed by stacking a plurality of insulating films formed using any of these materials.

For example, a 1500-nm-thick acrylic resin film may be formed as the planarization insulating film. The acrylic resin film can be formed in such a manner that an acrylic resin is applied by a coating method and then baked (e.g., at 250° C. for one hour in a nitrogen atmosphere).

Heat treatment may be performed after the planarization insulating film is formed. For example, heat treatment is performed at 250° C. for one hour in a nitrogen atmosphere.

As described above, heat treatment may be performed after formation of the transistor 440. Heat treatment may be performed more than once.

Consequently, a highly reliable semiconductor device which includes the transistor 440 including the oxide semiconductor film 403 and having stable electric characteristics can be provided. Further, the highly reliable semiconductor device can be manufactured at a high yield, whereby high productivity is achieved.

FIG. 11 shows results of secondary ion mass spectrometry (SIMS), where the chlorine concentration in an oxide semiconductor film of a transistor fabricated without performing cleaning treatment was measured. The transistor used for the measurement is formed using the same material and method as the transistor 440 of this embodiment, except for not being subjected to cleaning treatment and being provided with a protective insulating film which is formed using a silicon oxynitride film. Note that a region where an insulating layer functioning as a channel protective film is not provided was subjected to the measurement. The measurement was performed from a surface of the protective insulating film toward the depth direction. In the measurement region, the silicon oxynitride film which is the protective insulating film (thickness: 400 nm), an IGZO film which is an oxide semiconductor film, and a silicon oxynitride film which is a gate insulating film are stacked.

It is found from the measurement results in FIG. 11 that the chlorine concentration in the IGZO film that is the oxide semiconductor film (in the vicinity of the interface thereof) is higher than 1×10¹⁹/cm³. On the other hand, the chlorine concentration in the silicon oxynitride film that is the gate insulating film located directly under the IGZO film is lower than or equal to 1×10¹⁶/cm³. The chlorine concentration in the silicon oxynitride film that is the protective insulating film, which is formed after the etching step, is also around 1×10¹⁶/cm³. Since a gas containing chlorine is not used in the formation or the like of the protective insulating film and a measurement limit (a background level) of chlorine is about 4×10¹⁶/cm³, it is suggested that the chlorine concentration in the protective insulating film be under the measurement limit of SIMS. According to these results and consideration, the chlorine concentration in the IGZO film is obviously higher than those in the gate insulating film and the protective insulating film, indicating that the oxide semiconductor film that is not subjected to cleaning treatment contains chlorine. Further, consideration based on these measurement results naturally leads to a presumption that the chlorine concentration in the oxide semiconductor film after being subjected to cleaning treatment is lower than 1×10¹⁹/cm³ according to SIMS because chlorine that is attached to the oxide semiconductor film is removed by the cleaning treatment.

Embodiment 2

In this embodiment, another embodiment of a semiconductor device and a method for manufacturing the semiconductor device will be described with reference to FIGS. 3A and 3B and FIGS. 4A to 4D. The same portion as or a portion having a function similar to those in the above embodiment can be formed in a manner similar to that described in the above embodiment, and also the steps similar to those in the above embodiment can be performed in a manner similar to that described in the above embodiment, and repetitive description is omitted. In addition, detailed description of the same portions is not repeated.

A transistor 420 illustrated in FIGS. 3A and 3B is an example of a transistor which is one of bottom-gate transistors referred to as a channel-protective (channel-stop) transistor and is also referred to as an inverted-staggered transistor. FIG. 3A is a plan view, and FIG. 3B is a cross-sectional view taken along dashed-dotted line X2-Y2 in FIG. 3A.

As illustrated in FIG. 3B, which is a cross-sectional view in the channel length direction, a semiconductor device including the transistor 420 includes, over the substrate 400 having an insulating surface over which the insulating film 436 is provided, the gate electrode layer 401, the gate insulating film 402, the oxide semiconductor film 403 containing indium, an insulating layer 423, the source electrode layer 405 a, and the drain electrode layer 405 b.

The insulating layer 423 is provided over a region of the oxide semiconductor film 403 including at least the channel formation region, which overlaps with the gate electrode layer 401, and functions as a channel protective film. The insulating layer 423 includes an opening which reaches the oxide semiconductor film 403 and whose inner wall is covered with the source electrode layer 405 a or the drain electrode layer 405 b. Accordingly, the insulating layer 423 covers the periphery of the oxide semiconductor film 403, thus functioning also as an interlayer insulating film. Parasitic capacitance can be reduced by locating, in addition to the gate insulating film 402, the insulating layer 423 as an interlayer insulating film at the intersection of a gate wiring and a source wiring.

In the transistor 420, the oxide semiconductor film 403 is covered with the insulating layer 423, the source electrode layer 405 a, and the drain electrode layer 405 b.

The cross-sectional shape of the insulating layer 423 which is over and overlaps with the channel formation region, specifically, the cross-sectional shape (e.g., taper angle and thickness) of an end portion of the insulating layer 423 is adjusted, so that electric-field concentration which might occur in the vicinity of the end portion of the drain electrode layer 405 b can be reduced and degradation of the switching characteristics of the transistor 420 can be suppressed.

Specifically, the cross-sectional shape of the insulating layer 423 which is over and overlaps with the channel formation region is set to a trapezoid or a triangle, and the taper angle of a lower end portion of the cross-sectional shape is set to 60° or less, preferably 45° or less, further preferably 30° or less. Setting the taper angle within such a range makes it possible to reduce the electric-field concentration which might occur in the vicinity of the end portion of the drain electrode layer 405 b when a high gate voltage is applied to the gate electrode layer 401.

The thickness of the insulating layer 423 which is over and overlaps with the channel formation region is less than or equal to 0.3 μm, preferably greater than or equal to 5 nm and less than or equal to 0.1 μm. Setting the thickness within such a range makes it possible to reduce the peak of electric-field intensity, or distribute the electric-field concentration so that the electric-field is concentrated in plural portions, consequently reducing the electric-field concentration which might occur in the vicinity of the end portion of the drain electrode layer 405 b.

FIGS. 4A to 4D illustrate an example of a method for manufacturing the semiconductor device including the transistor 420.

The insulating film 436 is formed over the substrate 400 having an insulating surface. In this embodiment, a stack of a 100-nm-thick silicon nitride film and a 150-nm-thick silicon oxide film, which are formed by a plasma CVD method, is used as the insulating film 436.

A conductive film is formed over the insulating film 436 and etched, whereby the gate electrode layer 401 is formed. In this embodiment, a 100-nm-thick tungsten film is formed by a sputtering method.

The gate insulating film 402 is formed over the gate electrode layer 401. In this embodiment, a 200-nm-thick silicon oxynitride film is formed by a high-density plasma CVD method.

The oxide semiconductor film 403 is formed over the gate insulating film 402. In this embodiment, a 35-nm-thick In—Ga—Zn-based oxide film (IGZO film) is formed as the oxide semiconductor film 403 by a sputtering method using a sputtering apparatus including an AC power supply device. In this embodiment, an In—Ga—Zn-based oxide target having an atomic ratio of In:Ga:Zn=1:1:1 (=⅓:⅓:⅓) is used. The deposition conditions are as follows: the atmosphere is oxygen and argon (the proportion of the oxygen flow rate is 50%), the pressure is 0.6 Pa, the electric power is 5 kW, and the substrate temperature is 170° C. The deposition rate under these deposition conditions is 16 nm/min.

Heat treatment may be performed on the oxide semiconductor film 403 in order to remove excess hydrogen (including water and a hydroxyl group) (to perform dehydration or dehydrogenation treatment). In this embodiment, the substrate is introduced into an electric furnace which is one of heat treatment apparatuses, and the oxide semiconductor film 403 is subjected to heat treatment at 450° C. in a nitrogen atmosphere for one hour and further to heat treatment at 450° C. in an atmosphere containing nitrogen and oxygen for one hour.

Next, the insulating layer 423 having openings 425 a and 425 b which reach the oxide semiconductor film 403 is formed over the oxide semiconductor film 403 (see FIG. 4A).

The insulating layer 423 can be formed by etching an insulating film which is formed by a plasma CVD method or a sputtering method. Inner walls of the openings 425 a and 425 b in the insulating layer 423 are tapered.

The insulating layer 423 is provided over a region of the oxide semiconductor film 403 including at least the channel formation region of the oxide semiconductor film 403, which overlaps with the gate electrode layer 401, and partly functions as a channel protective film.

In this embodiment, the channel formation region of the oxide semiconductor film 403 is covered with the insulating layer 423. An end portion of the oxide semiconductor film 403 is also covered with the insulating layer 423.

In this embodiment, a 200-nm-thick silicon oxide film is formed by a sputtering method as the insulating layer 423. In this embodiment, the taper angle of the lower end portion is set to 30° in the cross section of the insulating layer 423.

Heat treatment may be performed after the formation of the insulating layer 423. In this embodiment, heat treatment is performed at 300° C. for one hour in a nitrogen atmosphere.

Next, the conductive film 445 for forming the source electrode layer and the drain electrode layer (including a wiring formed of the same layer as the source electrode layer and the drain electrode layer) is formed so as to cover the gate electrode layer 401, the gate insulating film 402, the oxide semiconductor film 403, the insulating layer 423, and the inner walls of the openings 425 a and 425 b (see FIG. 4B).

Through a photolithography process, the resist masks 448 a and 448 b are formed over the conductive film 445 and selective etching is performed, so that the source electrode layer 405 a and the drain electrode layer 405 b are formed (see FIG. 4C). After formation of the source electrode layer 405 a and the drain electrode layer 405 b, the resist masks are removed.

The gas 447 containing chlorine is used for the etching of the conductive film 445. As the gas 447 containing chlorine, for example, a gas containing chlorine (Cl₂), boron trichloride (BCl₃), silicon tetrachloride (SiCl₄), carbon tetrachloride (CCl₄), or the like can be used.

As the etching method, a parallel-plate reactive ion etching (RIE) method or an inductively coupled plasma (ICP) etching method can be used. In order to etch the film into desired shapes, the etching condition (the amount of electric power applied to a coil-shaped electrode, the amount of electric power applied to an electrode on a substrate side, the temperature of the electrode on the substrate side, or the like) is adjusted as appropriate.

In this embodiment, a stack of a 100-nm-thick titanium film, a 400-nm-thick aluminum film, and a 100-nm-thick titanium film, which are formed by a sputtering method, is used as the conductive film 445. As the etching of the conductive film 445, the stack of the titanium film, the aluminum film, and the titanium film is etched by dry etching, whereby the source electrode layer 405 a and the drain electrode layer 405 b are formed.

In this embodiment, the upper titanium film and the aluminum film are etched under first etching conditions and then the lower titanium film is etched under second etching conditions. The first etching conditions are as follows: an etching gas (BCl₃:Cl₂=750 sccm:150 sccm) is used, the bias power is 1500 W, the ICP power is 0 W, and the pressure is 2.0 Pa. The second etching conditions are as follows: an etching gas (BCl₃:Cl₂=700 sccm:100 sccm) is used, the bias power is 750 W, the ICP power is 0 W, and the pressure is 2.0 Pa.

As described above, the gas 447 containing chlorine is used in the etching step for forming the source electrode layer 405 a and the drain electrode layer 405 b. However, when the oxide semiconductor film 403 containing indium is exposed to the gas 447 containing chlorine, reaction occurs therebetween to generate residues. Further, when the residues are scattered, the residues exist on the surface of the insulating layer 423 and in the vicinity thereof between the source electrode layer 405 a and the drain electrode layer 405 b, as well as on the oxide semiconductor film 403. The residues on the surface of the insulating layer 423 and in the vicinity thereof between the source electrode layer 405 a and the drain electrode layer 405 b cause degradation of electric characteristics of the transistor 420, such as an increase in leakage current. Further, chlorine contained in the gas (in some cases, also an element other than chlorine contained in the gas) might enter or be attached to the oxide semiconductor film 403 to adversely affect transistor characteristics.

The residue includes, for example, a compound containing indium and chlorine. The residue may further include a metal element other than indium contained in the oxide semiconductor film (e.g., gallium or zinc) or an element other than chlorine used in the gas containing chlorine (e.g., boron).

In this embodiment, the oxide semiconductor film 403 is not exposed to the gas 447 containing chlorine in the etching step using the gas 447 containing chlorine, because the conductive film 445 is provided so as to cover the insulating layer 423 and the inner walls of the openings 425 a and 425 b. This can prevent reaction between the gas 447 containing chlorine and the oxide semiconductor film 403 containing indium, thus preventing generation of residues.

Through the above process, the transistor 420 of this embodiment can be manufactured (see FIG. 4D).

An insulating film serving as a protective insulating film may be formed over the source electrode layer 405 a and the drain electrode layer 405 b.

The protective insulating film can be formed using a material and a method similar to those of the insulating layer 423. For example, a 400-nm-thick silicon oxynitride film is formed by a CVD method. Heat treatment may be performed after formation of the protective insulating film. For example, heat treatment is performed at 300° C. for one hour in a nitrogen atmosphere.

In addition, a planarization insulating film may be formed in order to reduce surface unevenness due to the transistor 420.

For example, a 1500-nm-thick acrylic resin film may be formed as the planarization insulating film over the protective insulating film. The acrylic resin film can be formed in such a manner that an acrylic resin is applied by a coating method and then baked (e.g., at 250° C. for one hour in a nitrogen atmosphere).

Heat treatment may be performed after the planarization insulating film is formed. For example, heat treatment is performed at 250° C. for one hour in a nitrogen atmosphere.

As described above, the surface of the insulating layer 423 and the vicinity thereof can be prevented from being contaminated by the residues, so that the chlorine concentration and the indium concentration on the surface of the insulating layer 423 can be lower than or equal to 1×10¹⁹/cm³ (preferably lower than or equal to 5×10¹⁸/cm³) and lower than or equal to 2×10¹⁹/cm³ (preferably lower than or equal to 5×10¹⁸/cm³), respectively, in the semiconductor device including the transistor 420, which is an inverted-staggered transistor having a bottom-gate structure. Further, since the oxide semiconductor film 403 is not exposed to the gas 447 containing chlorine, the chlorine concentration in the oxide semiconductor film 403 can be lower than or equal to 1×10¹⁹/cm³ (preferably lower than or equal to 5×10¹⁸/cm³).

Consequently, a highly reliable semiconductor device which includes the transistor 420 including the oxide semiconductor film 403 and having stable electric characteristics can be provided. Further, the highly reliable semiconductor device can be manufactured at a high yield, whereby high productivity is achieved.

Embodiment 3

A semiconductor device having a display function (also referred to as a display device) can be manufactured using the transistor described in Embodiment 1 or 2. Further, part or all of the driver circuitry which includes the transistor can be formed over a substrate where a pixel portion is formed, whereby a system-on-panel can be formed.

In FIG. 5A, a sealant 4005 is provided so as to surround a pixel portion 4002 provided over a first substrate 4001, and the pixel portion 4002 is sealed with a second substrate 4006. In FIG. 5A, a scan line driver circuit 4004 and a signal line driver circuit 4003 which are each formed using a single crystal semiconductor film or a polycrystalline semiconductor film over a substrate separately prepared are mounted in a region that is different from the region surrounded by the sealant 4005 over the first substrate 4001. A variety of signals and potentials are supplied to the signal line driver circuit 4003 and the scan line driver circuit 4004 each of which is separately formed, and the pixel portion 4002 from flexible printed circuits (FPCs) 4018 a and 4018 b.

In FIGS. 5B and 5C, the sealant 4005 is provided so as to surround the pixel portion 4002 and the scan line driver circuit 4004 which are provided over the first substrate 4001. The second substrate 4006 is provided over the pixel portion 4002 and the scan line driver circuit 4004. Consequently, the pixel portion 4002 and the scan line driver circuit 4004 are sealed together with a display element by the first substrate 4001, the sealant 4005, and the second substrate 4006. In FIGS. 5B and 5C, the signal line driver circuit 4003 which is formed using a single crystal semiconductor film or a polycrystalline semiconductor film over a substrate separately prepared is mounted in a region that is different from the region surrounded by the sealant 4005 over the first substrate 4001. In FIGS. 5B and 5C, a variety of signals and potentials are supplied to the signal line driver circuit 4003 which is separately formed, the scan line driver circuit 4004, and the pixel portion 4002 from an FPC 4018.

Although FIGS. 5B and 5C each illustrate an example in which the signal line driver circuit 4003 is formed separately and mounted on the first substrate 4001, one embodiment of the present invention is not limited to this structure. The scan line driver circuit may be formed separately and then mounted, or only part of the signal line driver circuit or part of the scan line driver circuit may be formed separately and then mounted.

Note that there is no particular limitation on a connection method of a separately formed driver circuit, and a chip on glass (COG) method, a wire bonding method, a tape automated bonding (TAB) method, or the like can be used. FIG. 5A illustrates an example in which the signal line driver circuit 4003 and the scan line driver circuit 4004 are mounted by a COG method. FIG. 5B illustrates an example in which the signal line driver circuit 4003 is mounted by a COG method. FIG. 5C illustrates an example in which the signal line driver circuit 4003 is mounted by a TAB method.

The display device includes a panel in which a display element is sealed, and a module in which an IC such as a controller or the like is mounted on the panel.

Note that a display device in this specification means an image display device, a display device, or a light source (including a lighting device). Further, the display device also includes the following modules in its category: a module to which a connector such as an FPC, a TAB tape, or a TCP is attached; a module having a TAB tape or a TCP at the tip of which a printed wiring board is provided; and a module in which an integrated circuit (IC) is directly mounted on a display element by a COG method.

The pixel portion and the scan line driver circuit provided over the first substrate include a plurality of transistors, and the transistor described in Embodiment 1 or 2 can be applied thereto.

As the display element provided in the display device, a liquid crystal element (also referred to as a liquid crystal display element) or a light-emitting element (also referred to as a light-emitting display element) can be used. The light-emitting element includes, in its category, an element whose luminance is controlled by a current or a voltage, and specifically includes, in its category, an inorganic electroluminescent (EL) element, an organic EL element, and the like. Further, a display medium whose contrast is changed by an electric effect, such as electronic ink, can be used.

Embodiments of the semiconductor device will be described with reference to FIGS. 5A to 5C, FIGS. 6A and 6B, and FIGS. 7A and 7B. FIGS. 7A and 7B correspond to cross-sectional views taken along line M-N in FIG. 5B.

As illustrated in FIGS. 5A to 5C and FIGS. 7A and 7B, the semiconductor device includes a connection terminal electrode 4015 and a terminal electrode 4016. The connection terminal electrode 4015 and the terminal electrode 4016 are electrically connected to a terminal included in the FPC 4018 through an anisotropic conductive film 4019.

The connection terminal electrode 4015 is formed using the same conductive film as a first electrode layer 4030, and the terminal electrode 4016 is formed using the same conductive film as gate electrode layers of transistors 4040 and 4011.

The pixel portion 4002 and the scan line driver circuit 4004 which are provided over the first substrate 4001 include a plurality of transistors. FIGS. 7A and 7B illustrate the transistor 4040 included in the pixel portion 4002 and the transistor 4011 included in the scan line driver circuit 4004. In FIG. 7A, an insulating film 4020 is provided over the transistors 4040 and 4011, and in FIG. 7B, an insulating film 4021 is further provided. Note that an insulating film 4023 is an insulating film serving as a base film.

The transistor described in Embodiment 1 or 2 can be applied to the transistors 4040 and 4011. In this embodiment, an example in which a transistor having a structure similar to that of the transistor 440 described in Embodiment 1 is used will be described. The transistors 4040 and 4011 are inverted-staggered transistors with a bottom-gate structure, in each of which an insulating layer functioning as a channel protective film is provided over an oxide semiconductor film.

In the transistors 4040 and 4011 each having a structure similar to that of the transistor 440 described in Embodiment 1, after formation of the source electrode layer and the drain electrode layer, a step of removing a residue on the surface of the insulating layer and in the vicinity thereof between the source electrode layer and the drain electrode layer is performed. The removal step can be performed by cleaning treatment using a solution or plasma treatment using a rare gas. For example, cleaning treatment using a dilute hydrofluoric acid solution, plasma treatment using argon, or the like can be suitably used.

Alternatively, a structure similar to that of the transistor 420 described in Embodiment 2 may be applied to the transistors 4040 and 4011. The transistor described in Embodiment 2 has a structure in which, for the purpose of preventing reaction between the gas containing chlorine and the oxide semiconductor film containing indium, the oxide semiconductor film containing indium is covered with the insulating layer and the conductive film so as not to be exposed to the gas containing chlorine in the etching step using the gas containing chlorine. Accordingly, the insulating layer functioning as a channel protective film is provided over a region of the oxide semiconductor film including at least the channel formation region, which overlaps with the gate electrode layer, and has an opening which reaches the oxide semiconductor film and whose inner wall is covered with the source electrode layer or the drain electrode layer.

Since the surface of the insulating layer and the vicinity thereof can be prevented from being contaminated by the residue, the chlorine concentration and the indium concentration on the surface of the insulating layer of each of the transistors 4040 and 4011 can be lower than or equal to 1×10¹⁹/cm³ (preferably lower than or equal to 5×10¹⁸/cm³) and lower than or equal to 2×10¹⁹/cm³ (preferably lower than or equal to 5×10¹⁸/cm³), respectively. Further, the chlorine concentration in the oxide semiconductor film can be lower than or equal to 1×10¹⁹/cm³ (preferably lower than or equal to 5×10¹⁸/cm³).

Thus, a highly reliable semiconductor device can be provided as any of the semiconductor devices, illustrated in FIGS. 5A to 5C and FIGS. 7A and 7B, which includes the transistors 4040 and 4011 of this embodiment each including the oxide semiconductor film and having stable electric characteristics. Further, such a highly reliable semiconductor device can be manufactured with a high yield, so that high productivity can be achieved.

A conductive layer may be further provided so as to overlap with the channel formation region in the oxide semiconductor film of the transistor 4011 for the driver circuit. By providing the conductive layer so as to overlap with the channel formation region in the oxide semiconductor film, the amount of change in the threshold voltage of the transistor 4011 between before and after a bias-temperature stress test (BT test) can be further reduced. The conductive layer may have the same potential as or a potential different from that of a gate electrode layer of the transistor 4011, and can function as a second gate electrode layer. The potential of the conductive layer may be GND or 0 V, or the conductive layer may be in a floating state.

In addition, the conductive layer has a function of blocking an external electric field, that is, a function of preventing an external electric field (particularly, a function of preventing static electricity) from affecting the inside (circuit portion including a transistor). The blocking function of the conductive layer can prevent fluctuation in the electrical characteristics of the transistor due to an influence of an external electric field such as static electricity.

The transistor 4040 included in the pixel portion 4002 is electrically connected to a display element to form a display panel. There is no particular limitation on the kind of display element as long as display can be performed, and a variety of kinds of display elements can be employed.

An example of a liquid crystal display device using a liquid crystal element as a display element is illustrated in FIG. 7A. In FIG. 7A, a liquid crystal element 4013 which is a display element includes the first electrode layer 4030, a second electrode layer 4031, and a liquid crystal layer 4008. Insulating films 4032 and 4033 serving as alignment films are provided so that the liquid crystal layer 4008 is sandwiched therebetween. The second electrode layer 4031 is provided on the second substrate 4006 side, and the first electrode layer 4030 and the second electrode layer 4031 are stacked with the liquid crystal layer 4008 provided therebetween.

A spacer 4035 is a columnar spacer which is obtained by selective etching of an insulating film, and is provided in order to control the thickness of the liquid crystal layer 4008 (cell gap). Alternatively, a spherical spacer may be used.

In the case where a liquid crystal element is used as the display element, a thermotropic liquid crystal, a low-molecular liquid crystal, a high-molecular liquid crystal, a polymer dispersed liquid crystal, a ferroelectric liquid crystal, an anti-ferroelectric liquid crystal, or the like can be used. Such a liquid crystal material (liquid crystal composition) exhibits a cholesteric phase, a smectic phase, a cubic phase, a chiral nematic phase, an isotropic phase, or the like depending on conditions.

Alternatively, a liquid crystal composition exhibiting a blue phase for which an alignment film is unnecessary may be used for the liquid crystal layer 4008. In this case, the liquid crystal layer 4008 is in contact with the first electrode layer 4030 and the second electrode layer 4031. A blue phase is one of liquid crystal phases, which is generated just before a cholesteric phase changes into an isotropic phase while the temperature of a cholesteric liquid crystal is increased. The blue phase can be exhibited using a liquid crystal composition which is a mixture of a liquid crystal and a chiral agent. To increase the temperature range where the blue phase is exhibited, a liquid crystal layer may be formed by adding a polymerizable monomer, a polymerization initiator, and the like to a liquid crystal composition exhibiting a blue phase and by performing polymer stabilization treatment. The liquid crystal composition exhibiting a blue phase has a short response time, and has optical isotropy, which contributes to the exclusion of the alignment process and reduction of viewing angle dependence. In addition, since an alignment film does not need to be provided and rubbing treatment is unnecessary, electrostatic discharge damage caused by the rubbing treatment can be prevented and defects and damage of the liquid crystal display device in the manufacturing process can be reduced. Thus, productivity of the liquid crystal display device can be improved. A transistor including an oxide semiconductor film has a possibility that the electric characteristics of the transistor may fluctuate significantly by the influence of static electricity and deviate from the designed range. Thus, it is more effective to use a liquid crystal composition exhibiting a blue phase for the liquid crystal display device including the transistor including an oxide semiconductor film.

The specific resistivity of the liquid crystal material is higher than or equal to 1×10⁹ Ω·cm, preferably higher than or equal to 1×10¹¹ Ω·cm, further preferably higher than or equal to 1×10¹² Ω·cm. Note that the specific resistivity in this specification is measured at 20° C.

The size of a storage capacitor formed in the liquid crystal display device is set considering the leakage current of the transistor provided in the pixel portion or the like so that charge can be held for a predetermined period. The size of the storage capacitor may be set considering the off-state current of the transistor or the like. By using the transistor including an oxide semiconductor film, which is disclosed in this specification, it is enough to provide a storage capacitor having a capacitance that is ⅓ or less, preferably ⅕ or less of liquid crystal capacitance of each pixel.

In the transistor including an oxide semiconductor film, which is disclosed in this specification, the current in an off state (off-state current) can be made small. Accordingly, an electric signal such as an image signal can be held for a longer period, and a writing interval can be set longer in an on state. Thus, the frequency of refresh operation can be reduced, which leads to an effect of suppressing power consumption.

The transistor including an oxide semiconductor film, which is disclosed in this specification, can have relatively high field-effect mobility and thus can operate at high speed. For example, when such a transistor which can operate at high speed is used for a liquid crystal display device, a switching transistor in a pixel portion and a driver transistor in a driver circuit portion can be formed over one substrate. That is, since a semiconductor device formed of a silicon wafer or the like is not additionally needed as a driver circuit, the number of components of the semiconductor device can be reduced. In addition, by using the transistor which can operate at high speed in the pixel portion, a high-quality image can be provided.

For the liquid crystal display device, a twisted nematic (TN) mode, an in-plane-switching (IPS) mode, a fringe field switching (FFS) mode, an axially symmetric aligned micro-cell (ASM) mode, an optical compensated birefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, an anti-ferroelectric liquid crystal (AFLC) mode, or the like can be used.

A normally black liquid crystal display device such as a transmissive liquid crystal display device utilizing a vertical alignment (VA) mode may be used. Some examples are given as the vertical alignment mode. For example, a multi-domain vertical alignment (MVA) mode, a patterned vertical alignment (PVA) mode, or an advanced super view (ASV) mode can be used. This embodiment can also be applied to a VA liquid crystal display device. The VA liquid crystal display device has a kind of form in which alignment of liquid crystal molecules of a liquid crystal display panel is controlled. In the VA liquid crystal display device, liquid crystal molecules are aligned in a vertical direction with respect to a panel surface when no voltage is applied to the display device. It is also possible to use a method called domain multiplication or multi-domain design, in which a pixel is divided into some regions (subpixels) and molecules are aligned in different directions in their respective regions.

In the display device, a black matrix (light-blocking layer), an optical member (optical substrate) such as a polarizing member, a retardation member, or an anti-reflection member, and the like are provided as appropriate. For example, circular polarization may be obtained by using a polarizing substrate and a retardation substrate. In addition, a backlight, a side light, or the like may be used as a light source.

As a display method in the pixel portion, a progressive method, an interlace method, or the like can be employed. Further, color elements controlled in a pixel at the time of color display are not limited to three colors: R, G, and B (R, G, and B correspond to red, green, and blue, respectively). For example, R, G, B, and W (W corresponds to white); or R, G, B, and one or more of yellow, cyan, magenta, and the like can be used. The sizes of display regions may differ between respective dots of color elements. Note that one embodiment of the invention disclosed herein is not limited to the application to a display device for color display; one embodiment of the invention disclosed herein can also be applied to a display device for monochrome display.

Alternatively, as the display element included in the display device, a light-emitting element utilizing electroluminescence can be used. Light-emitting elements utilizing electroluminescence are classified according to whether a light-emitting material is an organic compound or an inorganic compound. In general, the former is referred to as an organic EL element, and the latter is referred to as an inorganic EL element.

In an organic EL element, by application of voltage to a light-emitting element, electrons and holes are separately injected from a pair of electrodes into a layer containing a light-emitting organic compound, and current flows. The carriers (electrons and holes) are recombined, and thus, the light-emitting organic compound is excited. The light-emitting organic compound returns to a ground state from the excited state, thereby emitting light. Owing to such a mechanism, this light-emitting element is referred to as a current-excitation light-emitting element. In this embodiment, an organic EL element is used as a light-emitting element.

The inorganic EL elements are classified according to their element structures into a dispersion-type inorganic EL element and a thin-film inorganic EL element. A dispersion-type inorganic EL element has a light-emitting layer where particles of a light-emitting material are dispersed in a binder, and its light emission mechanism is donor-acceptor recombination type light emission that utilizes a donor level and an acceptor level. A thin-film inorganic EL element has a structure where a light-emitting layer is sandwiched between dielectric layers, which are further sandwiched between electrodes, and its light emission mechanism is localized type light emission that utilizes inner-shell electron transition of metal ions. Note that an example of an organic EL element as a light-emitting element is described here.

To extract light emitted from the light-emitting element, at least one of the pair of electrodes has a light-transmitting property. A transistor and the light-emitting element are formed over a substrate. The light-emitting element can have any of the following structures: a top emission structure in which light emission is extracted through a surface opposite to the substrate; a bottom emission structure in which light emission is extracted through a surface on the substrate side; and a dual emission structure in which light emission is extracted through the surface opposite to the substrate and the surface on the substrate side.

FIGS. 6A and 6B and FIG. 7B illustrate examples of a light-emitting device including a light-emitting element as a display element.

FIG. 6A is a plan view of a light-emitting device and FIG. 6B is a cross-sectional view taken along dashed-dotted lines V1-W1, V2-W2, and V3-W3 in FIG. 6A. Note that, an electroluminescent layer 542 and a second electrode layer 543 are not illustrated in the plan view in FIG. 6A.

The light-emitting device illustrated in FIGS. 6A and 6B includes, over a substrate 500 provided with an insulating film 501 functioning as a base film, a transistor 510, a capacitor 520, and an intersection 530 of wiring layers. The transistor 510 is electrically connected to a light-emitting element 540. Note that FIGS. 6A and 6B illustrate a bottom-emission light-emitting device in which light from the light-emitting element 540 is extracted through the substrate 500.

The transistor described in Embodiment 1 or 2 can be applied to the transistor 510. In this embodiment, an example in which a transistor having a structure similar to that of the transistor 420 described in Embodiment 2 is used will be described. The transistor 510 is an inverted-staggered transistor with a bottom-gate structure, in which an insulating layer functioning as a channel protective film is provided over an oxide semiconductor film.

The transistor 510 includes gate electrode layers 511 a and 511 b, a gate insulating film 502, an oxide semiconductor film 512, an insulating layer 503, and conductive layers 513 a and 513 b functioning as a source electrode layer and a drain electrode layer.

In the transistor 510 having a structure similar to that of the transistor 420 described in Embodiment 2, for the purpose of preventing reaction between the gas containing chlorine and the oxide semiconductor film 512 containing indium, the oxide semiconductor film 512 containing indium is covered with the insulating layer 503 and the conductive film which is to be a source electrode layer or a drain electrode layer so as not to be exposed to the gas containing chlorine in the etching step using the gas containing chlorine. Accordingly, the insulating layer 503 functioning as a channel protective film is provided over a region of the oxide semiconductor film 512 including at least the channel formation region, which overlaps with the gate electrode layers 511 a and 511 b, and has an opening which reaches the oxide semiconductor film 512 and whose inner wall is covered with the conductive layer 513 a or 513 b functioning as a source electrode layer or a drain electrode layer.

Alternatively, a structure similar to that of the transistor 440 described in Embodiment 1 may be applied to the transistor 510. In the transistor 510, after formation of the conductive layers 513 a and 513 b functioning as a source electrode layer and a drain electrode layer, a step of removing a residue on a surface of the insulating layer 503 and in the vicinity thereof between the conductive layers 513 a and 513 b functioning as a source electrode layer and a drain electrode layer is performed. The removal step can be performed by cleaning treatment using a solution or plasma treatment using a rare gas. For example, cleaning treatment using a dilute hydrofluoric acid solution, plasma treatment using argon, or the like can be suitably used.

Since the surface of the insulating layer 503 and the vicinity thereof can be prevented from being contaminated by the residue, the chlorine concentration and the indium concentration on the surface of the insulating layer 503 of the transistor 510 can be lower than or equal to 1×10¹⁹/cm³ (preferably lower than or equal to 5×10¹⁸/cm³) and lower than or equal to 2×10¹⁹/cm³ (preferably lower than or equal to 5×10¹⁸/cm³), respectively. Further, the chlorine concentration in the oxide semiconductor film 512 can be lower than or equal to 1×10¹⁹/cm³ (preferably lower than or equal to 5×10¹⁸/cm³).

Thus, a highly reliable semiconductor device can be provided as the semiconductor device, illustrated in FIGS. 6A and 6B, which includes the transistor 510 of this embodiment each including the oxide semiconductor film 512 and having stable electric characteristics. Further, such a highly reliable semiconductor device can be manufactured with a high yield, so that high productivity can be achieved.

The capacitor 520 includes conductive layers 521 a and 521 b, the gate insulating film 502, an oxide semiconductor film 522, and a conductive layer 523. The gate insulating film 502 and the oxide semiconductor film 522 are sandwiched between the conductive layer 523 and the conductive layers 521 a and 521 b, so that the capacitor is formed.

The intersection 530 of wiring layers is an intersection of a conductive layer 533 and the gate electrode layers 511 a and 511 b. The conductive layer 533 and the gate electrode layers 511 a and 511 b intersect with each other with the gate insulating film 502 and the insulating layer 503 provided therebetween. In the structure described in Embodiment 2, not only the gate insulating film 502 but also the insulating layer 503 can be provided between the conductive layer 533 and the gate electrode layers 511 a and 511 b at the intersection 530 of wiring layers; thus, parasitic capacitance between the conductive layer 533 and the gate electrode layers 511 a and 511 b can be reduced.

In this embodiment, a 30-nm-thick titanium film is used as the gate electrode layer 511 a and the conductive layer 521 a, and a 200-nm-thick copper thin film is used as the gate electrode layer 511 b and the conductive layer 521 b. Thus, the gate electrode layer has a stacked-layer structure of a titanium film and a copper thin film.

A 25-nm-thick IGZO film is used as the oxide semiconductor films 512 and 522.

An interlayer insulating film 504 is formed over the transistor 510, the capacitor 520, and the intersection 530 of wiring layers. Over the interlayer insulating film 504, a color filter layer 505 is provided in a region overlapping with the light-emitting element 540. An insulating film 506 functioning as a planarization insulating film is provided over the interlayer insulating film 504 and the color filter layer 505.

The light-emitting element 540 having a stacked-layer structure in which a first electrode layer 541, the electroluminescent layer 542, and the second electrode layer 543 are stacked in that order is provided over the insulating film 506. The first electrode layer 541 and the conductive layer 513 a are in contact with each other in an opening formed in the insulating film 506 and the interlayer insulating film 504, which reaches the conductive layer 513 a; thus the light-emitting element 540 and the transistor 510 are electrically connected to each other. Note that a partition 507 is provided so as to cover part of the first electrode layer 541 and the opening.

As the interlayer insulating film 504, a silicon oxynitride film having a thickness greater than or equal to 200 nm and less than or equal to 600 nm, which is formed by a plasma CVD method can be used. Further, a photosensitive acrylic film having a thickness of 1500 nm and a photosensitive polyimide film having a thickness of 1500 nm can be used as the insulating film 506 and the partition 507, respectively.

As the color filter layer 505, for example, a chromatic light-transmitting resin can be used. As such a chromatic light-transmitting resin, a photosensitive organic resin or a nonphotosensitive organic resin can be used. A photosensitive organic resin layer is preferably used, because the number of resist masks can be reduced, leading to simplification of a process.

Chromatic colors are colors except achromatic colors such as black, gray, and white. The color filter layer is formed using a material which transmits only light of the chromatic color. As chromatic color, red, green, blue, or the like can be used. Cyan, magenta, yellow, or the like may also be used. “Transmitting only light of the chromatic color” means that the light transmitted through the color filter layer has a peak at a wavelength of light of the chromatic color. The thickness of the color filter layer may be controlled to be optimal as appropriate in consideration of the relationship between the concentration of a coloring material to be contained and the transmittance of light. For example, the color filter layer 505 may have a thickness greater than or equal to 1500 nm and less than or equal to 2000 nm.

In the light-emitting device illustrated in FIG. 7B, a light-emitting element 4513 which is a display element is electrically connected to the transistor 4040 provided in the pixel portion 4002. A structure of the light-emitting element 4513 is not limited to the shown stacked-layer structure including the first electrode layer 4030, an electroluminescent layer 4511, and the second electrode layer 4031. The structure of the light-emitting element 4513 can be changed as appropriate depending on a direction in which light is extracted from the light-emitting element 4513, or the like.

Partitions 4510 and 507 can be formed using an organic insulating material or an inorganic insulating material. It is particularly preferable that the partitions 4510 and 507 be formed using a photosensitive resin material to have openings over the first electrode layers 4030 and 541, respectively. A sidewall of each opening is formed as a tilted surface with continuous curvature.

The electroluminescent layers 4511 and 542 may be formed using either a single layer or a plurality of layers stacked.

A protective film may be formed over the second electrode layer 4031 and the partition 4510 and over the second electrode layer 543 and the partition 507 in order to prevent entry of oxygen, hydrogen, moisture, carbon dioxide, or the like into the light-emitting elements 4513 and 540. As the protective film, a silicon nitride film, a silicon nitride oxide film, a DLC film, or the like can be formed.

Further, the light-emitting elements 4513 and 540 may be covered with respective layers containing an organic compound deposited by an evaporation method so that oxygen, hydrogen, moisture, carbon dioxide, or the like do not enter the light-emitting elements 4513 and 540.

In addition, in a space which is formed with the first substrate 4001, the second substrate 4006, and the sealant 4005, a filler 4514 is provided for sealing. It is preferable that elements be packaged (sealed) with a protective film (such as a laminate film or an ultraviolet curable resin film) or a cover material with high air-tightness and little degasification so that the elements are not exposed to the outside air, in this manner.

As the filler 4514, an ultraviolet curable resin or a thermosetting resin can be used as well as an inert gas such as nitrogen or argon. For example, polyvinyl chloride (PVC), acrylic, polyimide, an epoxy resin, a silicone resin, polyvinyl butyral (PVB), or ethylene vinyl acetate (EVA) can be used. For example, nitrogen is used as the filler.

In addition, as needed, an optical film such as a polarizing plate, a circularly polarizing plate (including an elliptically polarizing plate), a retardation plate (a quarter-wave plate or a half-wave plate), or a color filter may be provided as appropriate on a light-emitting surface of the light-emitting element. Further, the polarizing plate or the circularly polarizing plate may be provided with an anti-reflection film. For example, anti-glare treatment by which reflected light can be diffused by projections and depressions on the surface so as to reduce the glare can be performed.

Further, an electronic paper in which electronic ink is driven can be provided as the display device. The electronic paper is also referred to as an electrophoretic display device (an electrophoretic display) and is advantageous in that it has the same level of readability as plain paper, it has lower power consumption than other display devices, and it can be made thin and lightweight.

An electrophoretic display device can have various modes. An electrophoretic display device contains a plurality of microcapsules dispersed in a solvent or a solute, each microcapsule containing first particles which are positively charged and second particles which are negatively charged. By applying an electric field to the microcapsules, the particles in the microcapsules move in opposite directions to each other and only the color of the particles gathering on one side is displayed. Note that the first particles and the second particles each contain pigment and do not move without an electric field. Moreover, the first particles and the second particles have different colors (which may be colorless).

Thus, an electrophoretic display device is a display device that utilizes a so-called dielectrophoretic effect by which a substance having a high dielectric constant moves to a high-electric field region.

A solution in which the above microcapsules are dispersed in a solvent is referred to as electronic ink. This electronic ink can be printed on a surface of glass, plastic, cloth, paper, or the like. Furthermore, by using a color filter or particles that have a pigment, color display can also be achieved.

Note that the first particles and the second particles in the microcapsules may each be formed of a single material selected from a conductive material, an insulating material, a semiconductor material, a magnetic material, a liquid crystal material, a ferroelectric material, an electroluminescent material, an electrochromic material, and a magnetophoretic material, or formed of a composite material of any of these.

As the electronic paper, a display device using a twisting ball display system can be used. The twisting ball display system refers to a method in which spherical particles each colored in black and white are arranged between a first electrode layer and a second electrode layer which are electrode layers used for a display element, and a potential difference is generated between the first electrode layer and the second electrode layer to control orientation of the spherical particles, so that display is performed.

In FIGS. 5A to 5C, FIGS. 6A and 6B, and FIGS. 7A and 7B, a flexible substrate as well as a glass substrate can be used as any of the first substrates 4001 and 500 and the second substrate 4006. For example, a plastic substrate having a light-transmitting property or the like can be used. As the plastic substrate, a fiberglass-reinforced plastics (FRP) plate, a polyvinyl fluoride (PVF) film, a polyester film, or an acrylic resin film can be used. In the case where a light-transmitting property is not needed, a metal substrate (metal film) of aluminum, stainless steel, or the like may be used. For example, a sheet with a structure in which an aluminum foil is sandwiched between PVF films or polyester films can be used.

In this embodiment, an aluminum oxide film is used as the insulating film 4020. The insulating film 4020 can be formed by a sputtering method or a plasma CVD method.

The aluminum oxide film provided as the insulating film 4020 over the oxide semiconductor film has a high shielding effect (blocking effect) of preventing penetration of both oxygen and impurities such as hydrogen and moisture.

Thus, in and after the manufacturing process, the aluminum oxide film functions as a protective film for preventing entry of impurities such as hydrogen and moisture, which can cause a change of electric characteristics, into the oxide semiconductor film and release of oxygen, which is a main component material of the oxide semiconductor, from the oxide semiconductor film. This is why the aluminum oxide film is preferred to be used.

The insulating films 4021 and 506 serving as planarization insulating films can be formed using an organic material having heat resistance, such as an acrylic-, polyimide-, or benzocyclobutene-based resin, polyamide, or epoxy. Other than such organic materials, it is also possible to use a low-dielectric constant material (low-k material), a siloxane-based resin, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), or the like. Note that the insulating film may be formed by stacking a plurality of insulating films formed using any of these materials.

There is no particular limitation on the methods of forming the insulating films 4021 and 506, and the following method or tool (equipment) can be used depending on the material: a sputtering method, an SOG method, spin coating, dipping, spray coating, a droplet discharge method (such as an inkjet method), a printing method (such as screen printing or offset printing), a doctor knife, a roll coater, a curtain coater, a knife coater, or the like.

The display device displays an image by transmitting light from a light source or a display element. Thus, the substrate and the thin films such as the insulating film and the conductive film provided for the pixel portion where light is transmitted have light-transmitting properties with respect to light in the visible light wavelength range.

The first electrode layer and the second electrode layer (which may be called a pixel electrode layer, a common electrode layer, a counter electrode layer, or the like) for applying voltage to the display element may have light-transmitting properties or light-reflecting properties, which depends on the direction in which light is extracted, the position where the electrode layer is provided, the pattern structure of the electrode layer, and the like.

The first electrode layers 4030 and 541 and the second electrode layers 4031 and 543 can be formed using a light-transmitting conductive material such as indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, indium tin oxide, indium zinc oxide, indium tin oxide to which silicon oxide is added, or graphene.

The first electrode layers 4030 and 541 and the second electrode layers 4031 and 543 can be formed using one or plural kinds selected from a metal such as tungsten (W), molybdenum (Mo), zirconium (Zr), hafnium (Hf), vanadium (V), niobium (Nb), tantalum (Ta), chromium (Cr), cobalt (Co), nickel (Ni), titanium (Ti), platinum (Pt), aluminum (Al), copper (Cu), or silver (Ag); an alloy thereof; and a nitride thereof.

In this embodiment, since the light-emitting device illustrated in FIGS. 6A and 6B has a bottom-emission structure, the first electrode layer 541 has a light-transmitting property and the second electrode layer 543 has a light-reflecting property. Accordingly, in the case of using a metal film as the first electrode layer 541, the film is preferably thin enough to secure a light-transmitting property; and in the case of using a light-transmissive conductive film as the second electrode layer 543, a conductive film having a light-reflecting property is preferably stacked therewith.

A conductive composition containing a conductive high molecule (also referred to as a conductive polymer) can be used for the first electrode layers 4030 and 541 and the second electrode layers 4031 and 543. As the conductive high molecule, what is called a π-electron conjugated conductive polymer can be used. For example, polyaniline or a derivative thereof, polypyrrole or a derivative thereof, polythiophene or a derivative thereof, a copolymer of two or more of aniline, pyrrole, and thiophene or a derivative thereof can be given.

Since the transistor is easily broken by static electricity or the like, a protection circuit for protecting the driver circuit is preferably provided. The protection circuit is preferably formed using a nonlinear element.

As described above, by using the transistor described in Embodiment 1 or 2, a semiconductor device having a variety of functions can be provided.

The structures, methods, and the like described in this embodiment can be combined as appropriate with any of the structures, methods, and the like described in the other embodiments.

Embodiment 4

A semiconductor device having an image sensor function of reading information on an object can be manufactured using the transistor described in Embodiment 1 or 2.

FIG. 8A illustrates an example of a semiconductor device having an image sensor function. FIG. 8A is an equivalent circuit diagram of a photosensor, and FIG. 8B is a cross-sectional view of part of the photosensor.

One electrode of a photodiode 602 is electrically connected to a photodiode reset signal line 658, and the other electrode of the photodiode 602 is electrically connected to a gate of a transistor 640. One of a source and a drain of the transistor 640 is electrically connected to a photosensor reference signal line 672, and the other of the source and the drain of the transistor 640 is electrically connected to one of a source and a drain of a transistor 656. A gate of the transistor 656 is electrically connected to a gate signal line 659, and the other of the source and the drain thereof is electrically connected to a photosensor output signal line 671.

Note that in a circuit diagram in this specification, a transistor including an oxide semiconductor film is denoted by a symbol “OS” so that it can be identified as a transistor including an oxide semiconductor film. In FIG. 8A, the transistor 640 and the transistor 656 are each a transistor including an oxide semiconductor film, to which the transistor described in Embodiment 1 or 2 can be applied. Described in this embodiment is an example in which a transistor having a structure similar to that of the transistor 440 described in Embodiment 1 is used. The transistor 640 is an inverted-staggered transistor with a bottom-gate structure, in which an insulating layer functioning as a channel protective film is provided over an oxide semiconductor film.

FIG. 8B is a cross-sectional view of the photodiode 602 and the transistor 640 in the photosensor. The transistor 640 and the photodiode 602 functioning as a sensor are provided over a substrate 601 (TFT substrate) having an insulating surface. A substrate 613 is provided over the photodiode 602 and the transistor 640 with the use of an adhesive layer 608.

An insulating film 631, an interlayer insulating film 633, and an interlayer insulating film 634 are provided over the transistor 640. The photodiode 602 is provided over the interlayer insulating film 633. In the photodiode 602, a first semiconductor film 606 a, a second semiconductor film 606 b, and a third semiconductor film 606 c are sequentially stacked from the interlayer insulating film 633 side, between an electrode layer 642 formed over the interlayer insulating film 634 and electrode layers 641 a and 641 b formed over the interlayer insulating film 633.

The electrode layer 641 b is electrically connected to a conductive layer 643 formed over the interlayer insulating film 634, and the electrode layer 642 is electrically connected to a conductive layer 645 through the electrode layer 641 a. The conductive layer 645 is electrically connected to the gate electrode layer of the transistor 640, and the photodiode 602 is electrically connected to the transistor 640.

Here, a pin photodiode in which a semiconductor film having p-type conductivity as the first semiconductor film 606 a, a high-resistance semiconductor film (i-type semiconductor film) as the second semiconductor film 606 b, and a semiconductor film having n-type conductivity as the third semiconductor film 606 c are stacked is illustrated as an example.

The first semiconductor film 606 a is a p-type semiconductor film and can be formed using an amorphous silicon film containing an impurity element imparting p-type conductivity. The first semiconductor film 606 a is formed by a plasma CVD method with the use of a semiconductor source gas containing an impurity element belonging to Group 13 (e.g., boron (B)). As the semiconductor source gas, silane (SiH₄) may be used. Alternatively, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄, or the like may be used. Further alternatively, an amorphous silicon film which does not contain an impurity element may be formed, and then an impurity element may be introduced into the amorphous silicon film by a diffusion method or an ion implantation method. Heating or the like may be conducted after introducing the impurity element by an ion implantation method or the like to diffuse the impurity element. In this case, as a method for forming the amorphous silicon film, an LPCVD method, a vapor deposition method, a sputtering method, or the like may be used. The first semiconductor film 606 a is preferably formed to have a thickness greater than or equal to 10 nm and less than or equal to 50 nm.

The second semiconductor film 606 b is an i-type semiconductor film (intrinsic semiconductor film) and is formed using an amorphous silicon film. As for formation of the second semiconductor film 606 b, an amorphous silicon film is formed by a plasma CVD method with the use of a semiconductor source gas. As the semiconductor source gas, silane (SiH₄) may be used. Alternatively, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄, or the like may be used. The second semiconductor film 606 b may be formed by an LPCVD method, a vapor deposition method, a sputtering method, or the like. The second semiconductor film 606 b is preferably formed to have a thickness greater than or equal to 200 nm and less than or equal to 1000 nm.

The third semiconductor film 606 c is an n-type semiconductor film and is formed using an amorphous silicon film containing an impurity element imparting n-type conductivity. The third semiconductor film 606 c is formed by a plasma CVD method with the use of a semiconductor source gas containing an impurity element belonging to Group 15 (e.g., phosphorus (P)). As the semiconductor source gas, silane (SiH₄) may be used. Alternatively, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄, or the like may be used. Further alternatively, an amorphous silicon film which does not contain an impurity element may be formed, and then an impurity element may be introduced into the amorphous silicon film by a diffusion method or an ion implantation method. Heating or the like may be conducted after introducing the impurity element by an ion implantation method or the like to diffuse the impurity element. In this case, as a method for forming the amorphous silicon film, an LPCVD method, a vapor deposition method, a sputtering method, or the like may be used. The third semiconductor film 606 c is preferably formed to have a thickness greater than or equal to 20 nm and less than or equal to 200 nm.

The first semiconductor film 606 a, the second semiconductor film 606 b, and the third semiconductor film 606 c are not necessarily formed using an amorphous semiconductor, and may be formed using a polycrystalline semiconductor or a microcrystalline semiconductor (semi-amorphous semiconductor: SAS).

The mobility of holes generated by the photoelectric effect is lower than the mobility of electrons. Thus, a pin photodiode has better characteristics when a surface on the p-type semiconductor film side is used as a light-receiving plane. Here, an example in which light received by the photodiode 602 from a surface of the substrate 601, over which the pin photodiode is formed, is converted into electric signals is described. Further, light from the semiconductor film having a conductivity type opposite to that of the semiconductor film on the light-receiving plane is disturbance light; thus, the electrode layer is preferably formed using a light-blocking conductive film. Note that a surface on the n-type semiconductor film side can alternatively be used as the light-receiving plane.

With the use of an insulating material, the insulating film 631, the interlayer insulating film 633 and the interlayer insulating film 634 can be formed, depending on the material, by a sputtering method, a plasma CVD method, an SOG method, spin coating, dipping, spray coating, a droplet discharge method (such as an inkjet method), a printing method (such as screen printing or offset printing), or the like.

The insulating film 631 can be formed using an inorganic insulating film. For example, and the insulating film 631 can have a single-layer structure or a stacked-layer structure including any of oxide insulating films such as a silicon oxide layer, a silicon oxynitride layer, an aluminum oxide layer, and an aluminum oxynitride layer; and nitride insulating films such as a silicon nitride layer, a silicon nitride oxide layer, an aluminum nitride layer, and an aluminum nitride oxide layer.

In this embodiment, an aluminum oxide film is used as the insulating film 631. The insulating film 631 can be formed by a sputtering method or a plasma CVD method.

The aluminum oxide film provided as the insulating film 631 over the oxide semiconductor film has a high shielding effect (blocking effect) of preventing penetration of both oxygen and impurities such as hydrogen and moisture.

Thus, in and after the manufacturing process, the aluminum oxide film functions as a protective film for preventing entry of impurities such as hydrogen and moisture, which can cause a change of electric characteristics, into the oxide semiconductor film and release of oxygen, which is a main component material of the oxide semiconductor, from the oxide semiconductor film. This is why the aluminum oxide film is preferred to be used.

To reduce surface roughness, an insulating film functioning as a planarization insulating film is preferably used as each of the interlayer insulating films 633 and 634. For the interlayer insulating films 633 and 634, for example, an organic insulating material having heat resistance, such as polyimide, acrylic resin, a benzocyclobutene-based resin, polyamide, or an epoxy resin, can be used. Other than such organic insulating materials, it is possible to use a single layer or stacked layers of a low-dielectric constant material (low-k material), a siloxane-based resin, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), or the like.

With detection of light that enters the photodiode 602, information on an object to be detected can be read. Note that a light source such as a backlight can be used at the time of reading information on an object to be detected.

In the transistor 640 having a structure similar to that of the transistor 440 described in Embodiment 1, after formation of the source electrode layer and the drain electrode layer, a step of removing a residue on the surface of the insulating layer and in the vicinity thereof between the source electrode layer and the drain electrode layer is performed. The removal step can be performed by cleaning treatment using a solution or plasma treatment using a rare gas. For example, cleaning treatment using a dilute hydrofluoric acid solution, plasma treatment using argon, or the like can be suitably used.

Alternatively, a structure similar to that of the transistor 420 described in Embodiment 2 may be applied to the transistor 640. The transistor described in Embodiment 2 has a structure in which, for the purpose of preventing reaction between the gas containing chlorine and the oxide semiconductor film containing indium, the oxide semiconductor film containing indium is covered with the insulating layer and the conductive film so as not to be exposed to the gas containing chlorine in the etching step using the gas containing chlorine. Accordingly, the insulating layer functioning as a channel protective film is provided over a region of the oxide semiconductor film including at least the channel formation region, which overlaps with the gate electrode layer, and has an opening which reaches the oxide semiconductor film and whose inner wall is covered with the source electrode layer or the drain electrode layer.

Since the surface of the insulating layer and the vicinity thereof can be prevented from being contaminated by the residue, the chlorine concentration and the indium concentration on the surface of the insulating layer of the transistor 640 can be lower than or equal to 1×10¹⁹/cm³ (preferably lower than or equal to 5×10¹⁸/cm³) and lower than or equal to 2×10¹⁹/cm³ (preferably lower than or equal to 5×10¹⁸/cm³), respectively. Further, the chlorine concentration in the oxide semiconductor film can be lower than or equal to 1×10¹⁹/cm³ (preferably lower than or equal to 5×10¹⁸/cm).

Thus, a highly reliable semiconductor device including the transistor 640 of this embodiment including the oxide semiconductor film and having stable electric characteristics can be provided. Further, a highly reliable semiconductor device can be manufactured with a high yield, so that high productivity can be achieved.

The structures, methods, and the like described in this embodiment can be combined as appropriate with any of the structures, methods, and the like described in the other embodiments.

Embodiment 5

A semiconductor device disclosed in this specification can be applied to a variety of electronic devices (including game machines). Examples of electronic devices include a television set (also referred to as a television or a television receiver), a monitor of a computer or the like, cameras such as a digital camera and a digital video camera, a digital photo frame, a mobile phone, a portable game machine, a portable information terminal, an audio reproducing device, a game machine (e.g., a pachinko machine or a slot machine), a game console, and the like. Specific examples of such electronic devices are illustrated in FIGS. 9A to 9C.

FIG. 9A illustrates a table 9000 having a display portion. In the table 9000, a display portion 9003 is incorporated in a housing 9001 and an image can be displayed on the display portion 9003. Note that the housing 9001 is supported by four leg portions 9002. Further, a power cord 9005 for supplying power is provided for the housing 9001.

The semiconductor device described in any of Embodiments 1 to 4 can be used for the display portion 9003 so that the electronic device can have a high reliability.

The display portion 9003 has a touch-input function. When a user touches displayed buttons 9004 which are displayed on the display portion 9003 of the table 9000 with his/her finger or the like, the user can carry out operation of the screen and input of information. Further, when the table may be made to communicate with home appliances or control the home appliances, the display portion 9003 may function as a control device which controls the home appliances by operation on the screen. For example, with use of the semiconductor device having an image sensor described in Embodiment 3, the display portion 9003 can function as a touch panel.

Further, the screen of the display portion 9003 can be placed perpendicular to a floor with a hinge provided for the housing 9001; thus, the table 9000 can also be used as a television set. A television set with a large screen takes up too much space that is available in a small room. However, with a table having a display portion therein, it is possible to make the use of the space in the room.

FIG. 9B illustrates a television set 9100. In the television set 9100, a display portion 9103 is incorporated in a housing 9101 and an image can be displayed on the display portion 9103. Note that the housing 9101 is supported by a stand 9105 here.

The television set 9100 can be operated with an operation switch of the housing 9101 or a separate remote controller 9110. Channels and volume can be controlled with operation keys 9109 of the remote controller 9110 so that an image displayed on the display portion 9103 can be controlled. Further, the remote controller 9110 may be provided with a display portion 9107 for displaying data output from the remote controller 9110.

The television set 9100 illustrated in FIG. 9B is provided with a receiver, a modem, and the like. With the receiver, the television set 9100 can receive a general television broadcast. Further, when the television set 9100 is connected to a communication network with or without wires connection via the modem, one-way (from a transmitter to a receiver) or two-way (between a transmitter and a receiver or between receivers) data communication can be performed.

The semiconductor device described in any of Embodiments 1 to 4 can be used for the display portions 9103 and 9107 so that the television set and the remote controller can have a high reliability.

FIG. 9C illustrates a computer which includes a main body 9201, a housing 9202, a display portion 9203, a keyboard 9204, an external connection port 9205, a pointing device 9206, and the like. The computer includes a semiconductor device manufactured in accordance with one embodiment of the present invention for the display portion 9203. When the semiconductor device described in the above embodiment is used, the computer can have a high reliability.

FIGS. 10A and 10B illustrate a tablet terminal that can be folded. In FIG. 10A, the tablet terminal is opened, and includes a housing 9630, a display portion 9631 a, a display portion 9631 b, a display-mode switching button 9034, a power button 9035, a power-saving-mode switching button 9036, a clip 9033, and an operation button 9038.

The semiconductor device described in any of Embodiments 1 to 4 can be used for the display portion 9631 a and the display portion 9631 b so that the tablet terminal can have a high reliability.

A touch panel area 9632 a can be provided in part of the display portion 9631 a, in which area, data can be input by touching displayed operation keys 9638. Note that half of the display portion 9631 a has only a display function and the other half has a touch panel function. However, one embodiment of the present invention is not limited to this structure, and the whole display portion 9631 a may have a touch panel function. For example, the display portion 9631 a can display a keyboard in the whole region to be used as a touch panel, and the display portion 9631 b can be used as a display screen.

A touch panel area 9632 b can be provided in part of the display portion 9631 b like in the display portion 9631 a. When a keyboard display switching instruction portion 9639 displayed on the touch panel is touched with a finger, a stylus, or the like, a keyboard can be displayed on the display portion 9631 b.

The touch panel area 9632 a and the touch panel area 9632 b can be controlled by touch input at the same time.

The display-mode switching button 9034 allows switching between a landscape mode and a portrait mode, color display and black-and-white display, and the like. The power-saving-mode switching button 9036 allows optimizing the display luminance in accordance with the amount of external light in use which is detected by an optical sensor incorporated in the tablet terminal. In addition to the optical sensor, another detecting device such as a sensor for detecting inclination, like a gyroscope or an acceleration sensor, may be incorporated in the tablet terminal.

Although the display portion 9631 a and the display portion 9631 b have the same display area in FIG. 10A, one embodiment of the present invention is not limited to this example. The display portion 9631 a and the display portion 9631 b may have different areas or different display quality. For example, higher definition images may be displayed on one of the display portions 9631 a and 9631 b.

FIG. 10B illustrates the tablet terminal folded, which includes the housing 9630, a solar battery 9633, a charge and discharge control circuit 9634, a battery 9635, and a DCDC converter 9636. Note that FIG. 10B shows an example in which the charge and discharge control circuit 9634 includes the battery 9635 and the DCDC converter 9636.

Since the tablet terminal can be folded, the housing 9630 can be closed when not in use. Thus, the display portions 9631 a and 9631 b can be protected, which makes it possible to provide a tablet terminal with high durability and improved reliability for long-term use.

The tablet terminal illustrated in FIGS. 10A and 10B can have other functions such as a function of displaying a variety of kinds of data (e.g., a still image, a moving image, and a text image), a function of displaying a calendar, a date, the time, or the like on the display portion, a touch-input function of operating or editing the data displayed on the display portion by touch input, and a function of controlling processing by a variety of kinds of software (programs).

The solar battery 9633, which is attached on the surface of the tablet terminal, supplies electric power to a touch panel, a display portion, an image signal processor, and the like. Note that a structure in which the solar battery 9633 is provided on one or both surfaces of the housing 9630 enables the battery 9635 to be charged efficiently. The use of a lithium ion battery as the battery 9635 is advantageous in downsizing or the like.

The structure and operation of the charge and discharge control circuit 9634 illustrated in FIG. 10B are described with reference to a block diagram of FIG. 10C. FIG. 10C illustrates the solar battery 9633, the battery 9635, the DCDC converter 9636, a converter 9637, switches SW1 to SW3, and the display portion 9631. The battery 9635, the DCDC converter 9636, the converter 9637, and the switches SW1 to SW3 correspond to the charge and discharge control circuit 9634 in FIG. 10B.

First, description is made on an example of the operation in the case where power is generated by the solar battery 9633 using external light. The voltage of power generated by the solar battery is raised or lowered by the DCDC converter 9636 so that a voltage for charging the battery 9635 is obtained. When the power from the solar battery 9633 is used for the operation of the display portion 9631, the switch SW1 is turned on and the voltage of the power is raised or lowered by the converter 9637 to a voltage needed for operating the display portion 9631. When display is not performed on the display portion 9631, the switch SW1 is turned off and the switch SW2 is turned on so that the battery 9635 can be charged.

Although the solar battery 9633 is shown as an example of a charge means, there is no particular limitation on the charge means and the battery 9635 may be charged with another means such as a piezoelectric element or a thermoelectric conversion element (Peltier element). For example, the battery 9635 may be charged with a non-contact power transmission module that transmits and receives power wirelessly (without contact) to charge the battery or with a combination of other charging means.

The structures, methods, and the like described in this embodiment can be combined as appropriate with any of the structures, methods, and the like described in the other embodiments.

This application is based on Japanese Patent Application serial no. 2011-215441 filed with Japan Patent Office on Sep. 29, 2011, the entire contents of which are hereby incorporated by reference. 

What is claimed is:
 1. A method for manufacturing a semiconductor device, comprising the steps of: forming a gate electrode over an insulating surface; forming a gate insulating film over the gate electrode; forming an oxide semiconductor film over the gate insulating film; forming an insulating layer comprising a contact hole over the oxide semiconductor film so that the insulating layer has a side surface; forming a conductive film over and in contact with the oxide semiconductor film and the insulating layer; etching the conductive film with a gas to form a source electrode and a drain electrode so that each of the source electrode and the drain electrode has a side surface; and removing residues on the oxide semiconductor film and the insulating layer, wherein an angle between the side surface of the source electrode and a bottom surface of the source electrode is greater than an angle between the side surface of the insulating layer and a bottom surface of the insulating layer, wherein an angle between the side surface of the drain electrode and a bottom surface of the drain electrode is greater than an angle between the side surface of the insulating layer and the bottom surface of the insulating layer, and wherein the residues are formed by a reaction between the gas and the oxide semiconductor film.
 2. The method according to claim 1, wherein a cleaning treatment using a dilute hydrofluoric acid solution is performed as the removing step.
 3. The method according to claim 1, wherein a plasma treatment using a rare gas is performed as the removing step.
 4. The method according to claim 1, wherein each of the residues comprises a compound containing indium and chlorine.
 5. The method according to claim 1, wherein the conductive film comprises a plurality of metal layers.
 6. The method according to claim 1, wherein the insulating layer comprises a portion that is thinner than the source electrode and the drain electrode.
 7. The method according to claim 1, wherein the gas contains chlorine.
 8. A method for manufacturing a semiconductor device, comprising the steps of: forming a gate electrode over an insulating surface; forming a gate insulating film over the gate electrode; forming an oxide semiconductor film over the gate insulating film; forming an insulating layer comprising a contact hole over and in contact with the oxide semiconductor film so that the insulating layer has a side surface; forming a conductive film over and in contact with the oxide semiconductor film and the insulating layer; etching the conductive film with a gas to form a source electrode and a drain electrode so that each of the source electrode and the drain electrode has a side surface; and removing residues on the oxide semiconductor film and the insulating layer, wherein an angle between the side surface of the source electrode and a bottom surface of the source electrode is greater than an angle between the side surface of the insulating layer and a bottom surface of the insulating layer, wherein an angle between the side surface of the drain electrode and a bottom surface of the drain electrode is greater than an angle between the side surface of the insulating layer and the bottom surface of the insulating layer, and wherein the residues are formed by a reaction between the gas and the oxide semiconductor film.
 9. The method according to claim 8, wherein a cleaning treatment using a dilute hydrofluoric acid solution is performed as the removing step.
 10. The method according to claim 8, wherein a plasma treatment using a rare gas is performed as the removing step.
 11. The method according to claim 8, wherein each of the residues comprises a compound containing indium and chlorine.
 12. The method according to claim 8, wherein the conductive film comprises a plurality of metal layers.
 13. The method according to claim 8, wherein the insulating layer comprises a portion that is thinner than the source electrode and the drain electrode.
 14. The method according to claim 8, wherein the gas contains chlorine. 